virtio_net: Allow setting the MAC address of the NIC

Many physical NICs let the OS re-program the "hardware" MAC
address.  Virtual NICs should allow this too.

Signed-off-by: Alex Williamson <alex.williamson@hp.com>
Acked-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>