I'm having several other problems, some of which I solved, some of which I can't.
Running Virtual Machine Manager only shows LXC in the list, whereas running "sudo virt-manager" shows only QEMU/KVM in the list. Requiring to use sudo on every single run. This should be the fix but it's not solving the problem.. There's also the issue that if I check "Enable XML editing", close and re-open, it doesn't save the setting.
I've wasted many hours already; still can't get GPU passthrough to work, and still can't run virt-manager without sudo. But got Windows running with VirtIO, and got samba file sharing working.
Then there's the mouse/keyboard passthrough... if I set USB passthrough on the mouse, any way to switch back to Linux? Other than unplugging and replugging the mouse.
EDIT: LOL! Solved the 'sudo' problem. Just had to start without sudo and File | Add Connection and just click Connect on QEMU/KVM. Now it works, and I can enable XML editing. Remains the GPU passthrough thing to solve.