vhost-net: tell tap backend about the vnet endianness

The default behaviour for TAP/MACVTAP is to consider vnet as native endian.

This patch handles the cases when this is not true:
- virtio 1.0: always little-endian
- legacy cross-endian

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>