Draft: libvirt-executor improvements

Deployed on runner1.archlinux.org

TODO:

• default pool? It is apparently created by virt-manager, so just using images is fine in our use-case (TODO: do we need to create it?)
• https://gitlab.archlinux.org/klausenbusk/arch-boxes/-/jobs/274061
• create the image in the arch-boxes project? arch-boxes!200 (merged)
• free-page-reporting?
• Discard?
• Cleanup the commit messages