virtio-net: reorganize receive_filter()

Reorganize receive_filter to better handle the split between
unicast and multicast filtering.  This allows us to skip the
broadcast check on unicast packets and leads to more opportunities
for optimization.

Signed-off-by: Alex Williamson <alex.williamson@hp.com>
Signed-off-by: Mark McLoughlin <markmc@redhat.com>
