This dependency has been added in 65eae4d, when NixOS switched to
systemd, as a substitute for the previous udevtrigger and hasn't been
touched since. It's probably unneeded as the upstream unit[1] doesn't
do it and I haven't found any mention of any problem in NixOS or the
upstream issue trackers.
[1]: https://gitlab.com/libvirt/libvirt/-/blob/master/src/remote/libvirtd.service.in