virtio-net: introduce a new macaddr control

virtio-net: introduce a new macaddr control

In virtio-net guest driver, currently we write MAC address to
pci config space byte by byte, this means that we have an
intermediate step where mac is wrong. This patch introduced
a new control command to set MAC address, it's atomic.

VIRTIO_NET_F_CTRL_MAC_ADDR is a new feature bit for compatibility.

"mac" field will be set to read-only when VIRTIO_NET_F_CTRL_MAC_ADDR
is acked.

Signed-off-by: Amos Kong <akong@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>