In testing qemu-5.1rc2 on my Fedora 32 home system, I found that the Fedora rawhide package has broken out both the QXL display device and the USB redirect device into separate RPM modules:
qemu-device-display-qxl.x86_64 2:5.1.0-0.1.rc2.fc32 @@commandline
qemu-device-usb-redirect.x86_64 2:5.1.0-0.1.rc2.fc32 @@commandline
The upgrade from 5.0.0 to 5.1.0 does not treat these sub-packages as mandatory packages, therefore a straight upgrade of packages causes these domcapabilities to disappear.
If the user tries to use libvirt after this, then existing domains using QXL display device or USB redirect device will fail to start. If the user then investigates and realizes that they now need to install the above packages separately, they find that qemu-kvm recognizes the modules right away, but libvirt does not. This looks to be due to the libvirt domcapabilities cache?
The domcapabilities cache will be automatically updated only under certain conditions such as the qemu binary ctime changing - but that isn't triggering any action here? With the devices broken out into modules, such as the Fedora rawhide RPM .spec file is proposing, this allows the devices to be individually installed or uninstalled at any time, and causes libvirt domcapabilities cache to be out-of-date.
I was able to sometimes see it work by downgrading to qemu-5.0, upgrading to qemu-5.1rc2, and installing the device packages prior to calling "virsh domcapabilities" (or otherwise using them). I was also able to do the same by removing the libvirt cache files and restarted libvirtd service. Both of these are pretty non-obvious actions for a user.
I'm wondering how to codify this when I use it for real on a production system. The upgrade path here seems unreliable, especially given that libvirt domcapabilities cache may even persist across reboots? This means I need to be very careful about the procedure to upgrade, and also I need to make sure to never install or remove any of the device packages without checking the procedure. Ouch. :-(