_findLiveSnapshotSupport: do not assume that guest <features> exist

Change-Id: I8e6d1ae6d020bedcfa8f2986fce628714e877765
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28349
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>