Hi. After my deep testing in various combinations of hardware, i can confirm that there is a bug, when using the dual-channel memory mode, which leads to guest (win10, win 8.1 in my case) freezes on systems with B350, B450 chipset.
Tested on Gigabyte A320M-S2H, Gigabyte B450i AORUS Pro Wifi, Asus B450 Strix I Gaming with Ryzen 5 2400 (3400G) and Kingston, Corsair, Noname memory in various combinations. This problem solved by installing different memory modules of different sizes (8G+4G) and (or) modules with different timings (this disables dual channel mode)

And now... Steps to reproduce: 

1. Check your memory worked in dual-channel mode.
2. Install Ubuntu(20.04), ArchLinux (kernel 5.6.15 in my case, with ACS patch), Manjaro KDE (19.x), it does not matter...
3. Install libvirt, qemu, ovmf and other virtualization stuff.
4. Configure configs:

/etc/default/grub:
GRUB_CMDLINE_LINUX_DEFAULT="audit=0 loglevel=3 quiet amd_iommu=on amd_iommu=pt pcie_acs_override=downstream,multifunction isolcpus=2,6,3,7 default_hugepagesz=1G hugepagesz=1G hugepages=8"

/etc/modprobe.d/avic.conf:
options kvm-amd nested=0 avic=1 npt=1

/etc/modprobe.d/kvm.conf:
options kvm halt_poll_ns=0
options kvm report_ignored_msrs=0
options kvm ignore_msrs=1
options vfio_iommu_type1 allow_unsafe_interrupts=1

/etc/modprobe.d/kvm.conf:
softdep amdgpu pre: vfio-pci
softdep snd_hda_intel pre: vfio-pci
options vfio-pci ids=1002:67ff,1002:aae0,1022:15e0,1022:15e1
options vfio-pci disable_vga=1

/etc/libvirt/qemu/win10.xml
<domain type='kvm'>
  <name>win10</name>
  <uuid>b33a0bec-cd23-4cfb-b4fe</uuid>
  <memory unit='KiB'>8388608</memory>
  <currentMemory unit='KiB'>8388608</currentMemory>
  <memoryBacking>
    <hugepages/>
    <nosharepages/>
    <locked/>
    <allocation mode='immediate'/>
    <discard/>
  </memoryBacking>
  <vcpu placement='static' cpuset='2,6,3,7'>4</vcpu>
  <cputune>
    <vcpupin vcpu='0' cpuset='2'/>
    <vcpupin vcpu='1' cpuset='6'/>
    <vcpupin vcpu='2' cpuset='3'/>
    <vcpupin vcpu='3' cpuset='7'/>
  </cputune>
  <os>
    <type arch='x86_64' machine='pc-q35-5.0'>hvm</type>
    <loader readonly='yes' type='pflash'>/usr/share/ovmf/x64/OVMF_CODE-pure-efi.fd</loader>
    <nvram>/usr/share/ovmf/x64/OVMF_VARS-pure-efi-1280x1024.fd</nvram>
    <bootmenu enable='no'/>
    <smbios mode='host'/>
  </os>
  <features>
    <acpi/>
    <apic eoi="on"/>
    <hap state="on"/>
    <hyperv>
      <relaxed state="on"/>
      <vapic state="on"/>
      <vpindex state="on"/>
      <vendor_id state="on" value="err43fix"/>
    </hyperv>
    <kvm>
      <hidden state="on"/>
      <hint-dedicated state="on"/>
    </kvm>
    <ioapic driver="kvm"/>
  </features>
  <cpu mode="host-passthrough" check="none">
    <topology sockets="1" cores="2" threads="2"/>
    <cache mode="passthrough"/>
    <feature policy="require" name="invtsc"/>
    <feature policy="require" name="topoext"/>
    <feature policy="disable" name="monitor"/>
  </cpu>
  <clock offset="utc">
    <timer name="rtc" present="no" tickpolicy="catchup"/>
    <timer name="pit" present="no" tickpolicy="discard"/>
    <timer name="hpet" present="no"/>
    <timer name="tsc" present="yes" mode="native"/>
    <timer name="hypervclock" present="yes"/>
  </clock>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <pm>
    <suspend-to-mem enabled='no'/>
    <suspend-to-disk enabled='no'/>
  </pm>
  <devices>
    <emulator>/usr/bin/qemu-system-x86_64</emulator>
    <disk type='file' device='cdrom'>
      <driver name='qemu' type='raw'/>
      <target dev='sdb' bus='sata'/>
      <readonly/>
      <address type='drive' controller='0' bus='0' target='0' unit='1'/>
    </disk>
    <disk type='file' device='cdrom'>
      <driver name='qemu' type='raw'/>
      <target dev='sdd' bus='sata'/>
      <readonly/>
      <address type='drive' controller='0' bus='0' target='0' unit='3'/>
    </disk>
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none' io='native'/>
      <source dev='/dev/disk/by-id/ata-INTEL_SSDSC2KW512G8_PHLA91930167512DGN'/>
      <target dev='sda' bus='scsi'/>
      <boot order='1'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
    <controller type='usb' index='0' model='qemu-xhci' ports='15'>
      <address type='pci' domain='0x0000' bus='0x02' slot='0x00' function='0x0'/>
    </controller>
    <controller type='sata' index='0'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x1f' function='0x2'/>
    </controller>
    <controller type='pci' index='0' model='pcie-root'/>
    <controller type='pci' index='1' model='pcie-root-port'>
      <model name='pcie-root-port'/>
      <target chassis='1' port='0x10'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0' multifunction='on'/>
    </controller>
    <controller type='pci' index='2' model='pcie-root-port'>
      <model name='pcie-root-port'/>
      <target chassis='2' port='0x11'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x1'/>
    </controller>
    <controller type='pci' index='3' model='pcie-root-port'>
      <model name='pcie-root-port'/>
      <target chassis='3' port='0x12'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x2'/>
    </controller>
    <controller type='pci' index='4' model='pcie-root-port'>
      <model name='pcie-root-port'/>
      <target chassis='4' port='0x13'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x3'/>
    </controller>
    <controller type='pci' index='5' model='pcie-root-port'>
      <model name='pcie-root-port'/>
      <target chassis='5' port='0x14'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x4'/>
    </controller>
    <controller type='pci' index='6' model='pcie-root-port'>
      <model name='pcie-root-port'/>
      <target chassis='6' port='0x15'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x5'/>
    </controller>
    <controller type='pci' index='7' model='pcie-root-port'>
      <model name='pcie-root-port'/>
      <target chassis='7' port='0x16'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x6'/>
    </controller>
    <controller type='pci' index='8' model='pcie-root-port'>
      <model name='pcie-root-port'/>
      <target chassis='8' port='0x17'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x7'/>
    </controller>
    <controller type='pci' index='9' model='pcie-to-pci-bridge'>
      <model name='pcie-pci-bridge'/>
      <address type='pci' domain='0x0000' bus='0x07' slot='0x00' function='0x0'/>
    </controller>
    <controller type='pci' index='10' model='pcie-root-port'>
      <model name='pcie-root-port'/>
      <target chassis='10' port='0x18'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0' multifunction='on'/>
    </controller>
    <controller type='pci' index='11' model='pcie-root-port'>
      <model name='pcie-root-port'/>
      <target chassis='11' port='0x19'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x1'/>
    </controller>
    <controller type='pci' index='12' model='pcie-root-port'>
      <model name='pcie-root-port'/>
      <target chassis='12' port='0x1a'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x2'/>
    </controller>
    <controller type='virtio-serial' index='0'>
      <address type='pci' domain='0x0000' bus='0x03' slot='0x00' function='0x0'/>
    </controller>
    <controller type='scsi' index='0' model='virtio-scsi'>
      <address type='pci' domain='0x0000' bus='0x0a' slot='0x00' function='0x0'/>
    </controller>
    <interface type='bridge'>
      <mac address='52:54:00:7e:27:af'/>
      <source bridge='virbr0'/>
      <model type='virtio'/>
      <address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
    </interface>
    <channel type='spicevmc'>
      <target type='virtio' name='com.redhat.spice.0'/>
      <address type='virtio-serial' controller='0' bus='0' port='1'/>
    </channel>
    <channel type='unix'>
      <source mode='bind' path='/tmp/win10.agent'/>
      <target type='virtio' name='org.qemu.guest_agent.0'/>
      <address type='virtio-serial' controller='0' bus='0' port='2'/>
    </channel>
    <input type='mouse' bus='ps2'/>
    <input type='keyboard' bus='ps2'/>
    <graphics type='spice' port='5910' autoport='no' listen='127.0.0.1'>
      <listen type='address' address='127.0.0.1'/>
      <image compression='off'/>
    </graphics>
    <video>
      <model type='qxl' ram='65536' vram='65536' vgamem='16384' heads='1' primary='yes'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x0'/>
    </video>
    <hostdev mode='subsystem' type='pci' managed='yes'>
      <driver name='vfio'/>
      <source>
        <address domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
      </source>
      <rom bar='off'/>
      <address type='pci' domain='0x0000' bus='0x05' slot='0x00' function='0x0'/>
    </hostdev>
    <hostdev mode='subsystem' type='pci' managed='yes'>
      <source>
        <address domain='0x0000' bus='0x08' slot='0x00' function='0x3'/>
      </source>
      <rom bar='off'/>
      <address type='pci' domain='0x0000' bus='0x06' slot='0x00' function='0x0'/>
    </hostdev>
    <hostdev mode='subsystem' type='pci' managed='yes'>
      <source>
        <address domain='0x0000' bus='0x08' slot='0x00' function='0x4'/>
      </source>
      <rom bar='off'/>
      <address type='pci' domain='0x0000' bus='0x08' slot='0x00' function='0x0'/>
    </hostdev>
    <memballoon model='virtio'>
      <address type='pci' domain='0x0000' bus='0x04' slot='0x00' function='0x0'/>
    </memballoon>
    <shmem name='looking-glass'>
      <model type='ivshmem-plain'/>
      <size unit='M'>32</size>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x10' function='0x0'/>
    </shmem>
  </devices>
</domain>
	  
5. Install Windows 10 (17763 build due to low latency), windows 8.1..., it does not matter...
6. Install Steam or other game stuff. it does not matter... Play. Your system will certainly go to the black screen if you use AMD GPU (tested with RADEON RX560 2GB) or your drivers has been crashed if you use Nvidia GPU (1050Ti 4Gb) in my case.
7. Turn off your PC, remove one memory module and install other memory module, with another timings, another capacity. This magic disables dual channel memory support.
8. Turn on your PC, start yor Virtualized Game Machine and all worked without problem days and nights without reboots...

I hope someone doesn't get past this problem. Thanks in advance.

P.S.: Dual channel worked fine without problem on bare metal hardware.