virtio-net: broken RX filtering logic fixed

Upon processing of VIRTIO_NET_CTRL_MAC_TABLE_SET command
multicast list overwrites unicast list in mac_table.
This leads to broken logic for both unicast and multicast RX filtering.

Signed-off-by: Dmitry Fleytman <dfleytma@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>