When I want to create a new virtual machine in virt-manager either by importing it from QCOW2 image or installing it from ISO image, virt-manager doesn't display the appropriate OS name when it tries to autodetect the OS. If I try to set it manually, it displays only Trisquel and "Generic or unknown OS".

Libosinfo is a package that contains information that helps virt-manager initialize and configure virtual machines' hardware depending on its OS type. Currently libosinfo is unmaintained, it was updated the last time on 2019-04-30. In its description it says that it removes support for detection of non-FSDG operating systems which account for most of the operating systems today except the few ones endorsed by GNU.

Please also consider this:
https://www.mail-archive.com/virt-tools-list@redhat.com/msg16579.html
Use of virt-install without a detected guest OS, and without manually
specified --os-variant, is a never ending source of bugs and user
confusion and poorly performing VM configs.

This all means that it is not possible to create proper and bugless configuration for VMs on Parabola with the means of libvirt-related tools unless one needs to use only Trisquel as a VM. Trisquel is the only OS detected by virt-manager now. Obviously, people who need virt-manager will likely run proprietary operating systems with it as well as those that are largely built on free software. I personally do this in order to isolate my host OS from malware that proprietary systems ship with because I am forced to use proprietary systems and this is how I can isolate them from my main domain of digital life happening on host OS. I believe there are other people who wish to use Parabola in the same way as me.

I took a look at files required to build libosinfo for Parabola and it has libre.patch file in it. If someone wants to make virt-manager usable again, I think it should be quite easy to do by removing this patch if it removes support for proprietary operating systems.