Same here.
Xbuntu 16.10 64-bit.
libvirtd (libvirt) 2.1.0
qemu-x86_64 version 2.6.1 (Debian 1:2.6.1+dfsg-0ubuntu5.1)
Using VirtIO drivers for the virtual HDD
Confirmed kills on Win 7 x64 as well as Win 8.1 x64 guests.
Once Avast is installed the host machine (Laptop) will hang and will have to be killed with a hard shut-down (long pwr-button press).
After rebooting KVM host and trying to start the Windows guest everything freezes again.
This is probably nested virtualization issue. I agree this issue is critical, but it is a KVM issue. As workaround, please just disable nested virtualization support in KVM.
Thank you for verification sigbee. The Avast hypervisor will not try to use virtualization extensions of CPU when running under KVM. From next Avast release.