tests: Add a test for qemu self announcements

We now expose qemu_announce_self through QMP and HMP.  Add a test
with some very basic packet validation (make sure we get a RARP).

Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>