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: NCornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: NGreg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NCornelia Huck <cornelia.huck@de.ibm.com>

hw/virtio/dataplane/vring.c

@@ -153,7 +153,8 @@ bool vring_should_notify(VirtIODevice *vdev, Vring *vring)
         return true;
     }
 
-    return vring_need_event(vring_used_event(&vring->vr), new, old);
+    return vring_need_event(virtio_tswap16(vdev, vring_used_event(&vring->vr)),
+                            new, old);
 }
 
 
@@ -407,7 +408,8 @@ int vring_pop(VirtIODevice *vdev, Vring *vring,
     /* On success, increment avail index. */
     vring->last_avail_idx++;
     if (virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX)) {
-        vring_avail_event(&vring->vr) = vring->last_avail_idx;
+        vring_avail_event(&vring->vr) =
+            virtio_tswap16(vdev, vring->last_avail_idx);
     }
 
     return head;