vdpa: Allow vDPA to work on big-endian machine

vdpa: Allow vDPA to work on big-endian machine

Add .set_vnet_le() function that always returns success, assuming that
vDPA h/w always implements LE data format. Otherwise, QEMU disables vDPA and
outputs the message:
"backend does not support LE vnet headers; falling back on userspace virtio"

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Konstantin Shkolnyy <kshk@linux.ibm.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
(cherry picked from commit b027f55a994af885a7a498a40373a2dcc2d8b15e)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>