dpif-netdev: Change pmd selection order.

dpif-netdev: Change pmd selection order.

Up to his point rxqs are sorted by processing cycles they
consumed and assigned to pmds in a round robin manner.

Ian pointed out that on wrap around the most loaded pmd will be
the next one to be assigned an additional rxq and that it would be
better to reverse the pmd order when wraparound occurs.

In other words, change from assigning by rr to assigning in a forward
and reverse cycle through pmds.

Also, now that the algorithm has finalized, document an example.

Suggested-by: Ian Stokes <ian.stokes@intel.com>
Signed-off-by: Kevin Traynor <ktraynor@redhat.com>
Signed-off-by: Darrell Ball <dlu998@gmail.com>