virtio-net: MAC filter optimization

virtio-net: MAC filter optimization

The MAC filter table is received from the guest as two separate
buffers, one with unicast entries, the other with multicast
entries.  If we track the index dividing the two sets, we can
avoid searching the part of the table with the wrong type of
entries.

We could store this index as part of the save image, but its
trivially easy to discover it on load.

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