net: enable IFF_VNET_HDR on tap fds if available

net: enable IFF_VNET_HDR on tap fds if available

For now, we just add an empty header before writing and strip the header
after reading.

We really only want IFF_VNET_HDR when virtio_net is using it, but it
would significantly complicate matters to try and do that. There should
be little or no performance impact with always adding headers.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>