tap prepends a zeroed virtio-net header when writing a packet to a
tap with virtio-net header enabled but not in use. This only happens
when s->host_vnet_hdr_len == sizeof(struct virtio_net_hdr).
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>