vdsm hooks: Add missing "snapshot=no" attribute to "disk" elements.

Missing snapshot="no" attribute is leading live snapshot to fail.
This patch adds snapshot="no" attribute to vdsm hooks adding disks.

Change-Id: Id6422c45450ffc65e4fe84ab7d66a30a285dd42a
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1036111
Signed-off-by: Amador Pahim <apahim@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21913
Reviewed-by: Dan Kenigsberg <danken@redhat.com>