Don't show bonds without slaves

Add a property to BondedNics() to query slaves, and filter the
results if it doesn't have any.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1094878

Change-Id: Ia2a718a9733a8e0981b68b706787a10768bd8d82
Signed-off-by: Ryan Barry <rbarry@redhat.com>
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>