virtio-net: mac property is mandatory

Mac feature bit isn't going to work as all network cards already have a
'mac' property to set the mac address.  Remove it from mask and add in
get_features.

Reported-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/hw/virtio-net.c b/hw/virtio-net.c
index c2a389f..02d9180 100644
--- a/hw/virtio-net.c
+++ b/hw/virtio-net.c
@@ -151,6 +151,8 @@
 {
     VirtIONet *n = to_virtio_net(vdev);
 
+    features |= (1 << VIRTIO_NET_F_MAC);
+
     if (peer_has_vnet_hdr(n)) {
         tap_using_vnet_hdr(n->nic->nc.peer, 1);
     } else {