dpif-netdev: Allow multi-rx-queue, multi-pmd-thread configuration.

dpif-netdev: Allow multi-rx-queue, multi-pmd-thread configuration.

This commits adds the multithreading functionality to OVS dpdk
module.  Users are able to create multiple pmd threads and set
their cpu affinity via specifying the cpu mask string similar
to the EAL '-c COREMASK' option.

Also, the number of rx queues for each dpdk interface is made
configurable to help distribution of rx packets among multiple
pmd threads.

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