net/e1000: support xstats by ID
net/e1000: support xstats by ID

To achieve functionality of retrieving only specific statistics
given by application there are two new functions added:
eth_igb_xstats_get_by_ids() which retrieve
values of statistics specified by ids array
and eth_igb_xstats_get_names_by_ids() which retrieve
names of statistics specified by ids array.

Signed-off-by: Jacek Piasecki <5e1ddc2c85b7acbcba75e0e83b3bb6ce7fe01339@intel.com>
Signed-off-by: Kuba Kozak <67670b66df7eb69a86e2579475d68b34f383c70c@intel.com>
Acked-by: Harry van Haaren <b9ad04ed670c22cfa7f5f9ab17e311ca85d11a76@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk