Live Merge: Add liveMerge capability to vdsCaps

It is desirable for engine to know up front if a host can support live
merge so the UI can either enable or disable the 'Delete' snapshot
command based on presence of the feature.

Change-Id: Iac66c679166b5687ed3940e517fe6827fe10e258
Signed-off-by: Adam Litke <alitke@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28998
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>