dpif-netdev: Add dpif-netdev/pmd-stats-* appctl commands.

dpif-netdev: Add dpif-netdev/pmd-stats-* appctl commands.

These commands can be used to get packets and cycles counters on a pmd
thread basis.  They're useful to get a clearer picture about the
performance of the userspace datapath.

They export these pieces of information:

- A (per-thread) view of the caches hit rate. Hits in the exact match
  cache are reported separately from hits in the masked classifier
- A rough cycles count. This will allow to estimate the load of OVS and
  the polling overhead.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Ethan Jackson <ethan@nicira.com>