libvirt-guests: Weaken dependency on libvirtd

libvirt-guests: Weaken dependency on libvirtd

The Requires relationship is very strong, in that it prevents
a unit from running unless all the units it Requires are
running as well.

This turns out to be a problem because we want to be able to
restart libvirtd at any time without having libvirt-guests
suspend or shutdown running guests.

Turn the Requires relationship into a Wants relationship:
this way starting libvirt-guests will cause systemd to (attempt
to) start libvirtd as well, but stopping or restarting libvirtd
will not alter libvirt-guests' running state.