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 <jacekx.piasecki@intel.com>
Signed-off-by: Kuba Kozak <kubax.kozak@intel.com>
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>