systemd: Make @sockprefix@ optional

systemd: Make @sockprefix@ optional

For most services, the socket paths can be derived trivially from
the name of the daemon: for virtqemud, for example, they will be

  /run/libvirt/virtqemud-sock
  /run/libvirt/virtqemud-sock-ro
  /run/libvirt/virtqemud-admin-sock

libvirtd and virtproxyd are the exceptions, since their socket
paths will be

  /run/libvirt/libvirt-sock
  /run/libvirt/libvirt-sock-ro
  /run/libvirt/libvirt-admin-sock

So we still need to be able to provide a custom @sockprefix@ in
those cases, but in the most common scenario we can do away with
the requirement by introducing a sensible default.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>