ethdev: fetch extended statistics with integer ids

ethdev: fetch extended statistics with integer ids

The current extended ethernet statistics fetching involve doing several
string operations, which causes performance issues if there are lots of
statistics and/or network interfaces. This patch changes the xstats
functions to instead use a numeric identifier rather than a string, and
adds the ability to retrieve identifier-to-string mappings.

Signed-off-by: Remy Horton <remy.horton@intel.com>