dpif-netdev: Create multiple pmd threads by default.

dpif-netdev: Create multiple pmd threads by default.

With this commit, ovs by default will create one pmd thread
for each numa node and pin the pmd thread to available cpu
core on the numa node.

NON_PMD_CORE_ID (currently 0) is used to reserve a particular
cpu core for the I/O of all non-pmd threads.  No pmd thread
can be pinned to this reserved core.

As side-effects of this commit:

-  pmd thread will not be created, if there is no dpdk interface
   from the corresponding numa node added to ovs.

- the exact-match cache for non-pmd threads is removed from
  'struct dp_netdev'.  Instead, all non-pmd threads will use
  the exact-match cache defined in the 'struct dp_netdev_pmd_thread'
  for NON_PMD_CORE_ID.

- the rx packet processing functions are refactored to use
  'struct dp_netdev_pmd_thread' as input.

- the 'netdev_send()' function will be called with the proper
  queue id.

- both pmd and non-pmd threads can call the dpif_netdev_execute().
  so, use a per-thread key to help recognize the calling thread.

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