I installed it a couple years ago, when I was researching the various hypervisors. I remember that I couldn't get anywhere with it, but not details. Actually, it seems no more difficult to set up than does VirtualBox, but I just can't get a VM to work correctly using virt-manager.
After you mentioned it here, I decided to give it another try -- perhaps I'd given up too soon the last time. I decided to create a Q4OS VM, because I'd just done that again with VirtualBox, and the Plasma DE is my daily driver.
The first VM I created, I'd not noticed that it was defaulting to a 15GB system, and using 4GB of that for swap. When I noticed that during installation, I almost stopped and started again with more storage, but in the end, I went ahead with it. The result was a VM that prompted me with a logon screen, and after accepting my password, left me with a black screen and a locked-up system. Obviously, 15GB was enough to install the OS, but not enough to run it.
So I deleted that VM and started over, this time providing 32GB. That created a system with 8.2GB of swap. This time, the VM actually ran after the password was accepted, however it ran as 1024x768, the same size as the original "live" system that ran the installer. I used the Display and Monitor dialog from the System Settings to change that to 1920x1080. When I clicked [Apply], the image changed to that size and filled my laptop screen, but after about 500ms or so it reverted to 1024x768. The combobox in the settings dialog still indicated that resolution was set to 1920x1080, but that was incorrect.
So I deleted that VM and tried once more. I looked through the defaults for creating a VM, and made one change to do with the video manager, and decided to try UEFI this time. I gave the VM 4096 memory, 4 cores, and 32GB storage. Everything went exactly as it happened with the previous attempt. There was no way I could get a VM to display any larger than 1024x768.
At this point, I'm assuming that this is what I couldn't remember from the original time I tried to use virt-manager. After attempting three VMs this time, all failures, I think I've given it an honest try, and I'm not enthsiastic about working more with it. I was very happy with the way these VMs performed, but just could not live with a maximum display size that's not been popular since the 1990s.