Get multiple port stats simultaneously

Get multiple port stats simultaneously

JIRA: YARDSTICK-1220

Implemented new "multi port stats <port list>" command, which returns a
semi-colon-separated list of comma-separated values:
- port id
- total (at last_tsc) for rx_pkts, tx_pkts, no_mbufs, ierrors + imissed
- last_tsc

Change-Id: I8a8da79cb4ccb245ccb38b50b561da51a40a359d
Signed-off-by: Patrice Buriez <patrice.buriez@intel.com>
(cherry picked from commit a9d9979b1f5b7435552ae884da69220a42b81f81)
Signed-off-by: Emma Foley <emma.l.foley@intel.com>