Might be installed from Custom ISO but not reported with qemu guest agent, I don't want to inspect virtual machines manually over VNC
Discussion
they could also use that nixos-takeover script for Debian. used it once but it's a bit iffy
Won't impact system files and change what qemu guest agent reports?
Well like all of those takeover scripts it just changes the system files. The only special thing is that it autogenerates a configuration\.nix file with networking stuff and public keys.
Pretty neat though. Would be nice if NixOS devs decided to create their own cloudinit images.
P.S. You should take a look at this https://bsd-cloud-image.org/