netdev: Add function for configuring tx and rx queues.

netdev: Add function for configuring tx and rx queues.

This commit adds a new API to the 'struct netdev_class' which
allows user to configure the number of tx queues and rx queues
of 'netdev'.  Upcoming patches will use this function to set
multiple tx/rx queues when adding the netdev to dpif-netdev.

Currently, only netdev-dpdk module implements this function.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>