Don't make default network a conffile
This addresses #973489.
I've tested upgrades from 5.0.0-4 (buster) and 6.8.0-1 (bullseye), as well as upgrades from 6.9.0-2 to a made-up 6.9.0-3 and installation from scratch: everything seems to work correctly.
This MR builds on top of !76 (merged) for the same reason !77 (merged) does. The only changes that are specific to the default network are in abologna/libvirt@b07918c5, so you can look at that commit only and ignore the rest.