Adding libvirtd dependency when supervdsm starts up with systemd

libvirt service needs to be up and serving before supervdsm starts up.

Change-Id: I8794c90290bbe902b2e9d5ad95483dd57e63ad0b
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17195
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>