virtio_net: bigger header when VERSION_1 is set

With VERSION_1 virtio_net uses same header size
whether mergeable buffers are enabled or not.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>