dpif: Abstract translation from OpenFlow queue ID into ODP priority value.

When the QoS code was integrated, I didn't yet know how to abstract the
translation from a queue ID in an OpenFlow OFPAT_ENQUEUE action into a
priority value for an ODP ODPAT_SET_PRIORITY action.  This commit is a
first attempt that works OK for Linux, so far.  It's possible that in fact
this translation needs the 'netdev' as an argument too, but it's not needed
yet.

lib/dpif-netdev.c

@@ -1366,6 +1366,7 @@ const struct dpif_class dpif_netdev_class = {
     dpif_netdev_recv_set_mask,
     NULL,                       /* get_sflow_probability */
     NULL,                       /* set_sflow_probability */
+    NULL,                       /* queue_to_priority */
     dpif_netdev_recv,
     dpif_netdev_recv_wait,
 };