dataplane: fix cross-endian issues

dataplane: fix cross-endian issues

Accesses to vring_avail_event and vring_used_event must honor the queue
endianness.

This patch allows cross-endian setups to use dataplane (tested with ppc64
on ppc64le, and vice-versa).

Suggested-by: Cornelia Huck <cornelia.huck@de.ibm.com>
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>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>