netdev-dummy: Add a dummy queue.

netdev-dummy: Add a dummy queue.

Until now it's been pretty hard to properly test any of the queue support,
because the dummy network device doesn't have any queues.  By adding one
queue to the dummy network device (queue 0), we can get slightly higher
confidence that OVS queue support works correctly.  I suppose we could do
even better if we made the dummy network device support modifying the
queues, but that's a job for another day.

Signed-off-by: Ben Pfaff <blp@ovn.org>