Do not allow empty network names (bridged nor bridgeless)

It was possible to create empty network names that ended up in
libvirt networks called 'vdsm-'. This patch solved it returning
the appropriate error and not allowing empty bridge names in the
internal representation either.

Change-Id: I1d1e5d9fb284eb67936daf197e3025314a29f58b
Bug-Url: https://bugzilla.redhat.com/1078899
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26263
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26367