tools/virtio: fix byteswap logic

commit cf561f0d ("virtio: introduce
virtio_is_little_endian() helper") changed byteswap logic to
skip feature bit checks for LE platforms, but didn't
update tools/virtio, so vring_bench started failing.

Update the copy under tools/virtio/ (TODO: find a way to avoid this code
duplication).

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