virtlogd: Don't stop or restart along with libvirtd

virtlogd: Don't stop or restart along with libvirtd

Commit 839a060 tied the lifecycle of virtlogd more
closely to that of libvirtd. Unfortunately, while starting
virtlogd when libvirtd is started is definitely a good idea,
restarting virtlogd or shutting it down at any time outside
of system poweroff is not.

Revert part of that commit by removing the PartOf= lines,
meaning that only startup requests will be propagated from
libvirtd to virtlogd.

Resolves: https://bugzilla.redhat.com/1372576