I realize this isn't a Debian-specific question, but hoped someone might be able to help anyway.
I'm using virt-manger, and its related suite of stuff, to run KVM/QEMU VM's on a Debian Bullseye host, which has two 4K monitors. I am unable to get dual-head working at all, and I can't get 4K resolution working in the way I'm apparently supposed to.
I have a Debian Bullseye guest, with spice-vdagent and the QXL video drivers installed in it. The Spice user manual suggests that this is easy; you're supposed to use a Video QXL device and then everything should just work. But it doesn't. Even when I increase the video memory to 64MB I am unable to get 4K using Settings -> Display on the guest; it maxes out at something arbitrary-seeming thing a bit over 1980 x 1200. And regardless of resolution, I also do not have a second monitor available in the guest.
If I have a video device set to Virtio, then 4K works perfectly on the guest machine; if I go to Settings -> Display there, I can select 4096 x 2160, and I get lovely 4K video. However, dual-head does not work. If I try to edit the XML for the video device to 'heads="2"', there is no effect--at least, when I restart the guest and go to Settings -> Display, there isn't an option for selecting another monitor.
Could someone tell me what I'm supposed to be doing?