vm payload: custom volume labels

Change required for cloud-init integration: support custom volume labels
for vm payload.

Change-Id: I5247d8f4ee01c0c871912a7998bdf958fc71ddd1
Bug-Url: https://bugzilla.redhat.com/619761
Signed-off-by: Greg Padgett <gpadgett@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14347
Reviewed-by: Dan Kenigsberg <danken@redhat.com>