domain: select the cluster lock using makeClusterLock

In order to support different locking mechanisms (not only per-domain
format but also per-domain type) a new makeClusterLock method has been
introduced to select the appropriate cluster lock.

Change-Id: I78072254441335a420292af642985840e9b2ac68
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/10281
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>