For libxlDomainLookupByID and libxlDomainLookupByUUID let's
return a locked and referenced @vm object so that callers can
then use the common and more consistent virDomainObjEndAPI in
order to handle cleanup rather than needing to know that the
returned object is locked and calling virObjectUnlock.

The LookupByName already returns the ref counted and locked object,
so this will make things more consistent.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>
Reviewed-by: NJim Fehlig <jfehlig@suse.com>