virtio-net: enable mergeable receive buffers

When virtio-net was merged in from qemu-kvm.git, the VNET_HDR related
features were dropped from the code.

However, VIRTIO_NET_F_MRG_RXBUF appears to have accidentally been
dropped too. Re-instate that now.
Reported-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMark McLoughlin <markmc@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/virtio-net.c

@@ -119,6 +119,7 @@ static void virtio_net_reset(VirtIODevice *vdev)
 static uint32_t virtio_net_get_features(VirtIODevice *vdev)
 {
     uint32_t features = (1 << VIRTIO_NET_F_MAC) |
+                        (1 << VIRTIO_NET_F_MRG_RXBUF) |
                         (1 << VIRTIO_NET_F_STATUS) |
                         (1 << VIRTIO_NET_F_CTRL_VQ) |
                         (1 << VIRTIO_NET_F_CTRL_RX) |