ethdev: retrieve xstats by ID

ethdev: retrieve xstats by ID

Extended xstats API in ethdev library to allow grouping of stats
logically so they can be retrieved per logical grouping  managed
by the application.
Changed existing functions rte_eth_xstats_get_names and
rte_eth_xstats_get to use a new list of arguments: array of ids
and array of values. ABI versioning mechanism was used to
support backward compatibility.
Introduced two new functions rte_eth_xstats_get_all and
rte_eth_xstats_get_names_all which keeps functionality of the
previous ones (respectively rte_eth_xstats_get and
rte_eth_xstats_get_names) but use new API inside.

test-pmd: add support for new xstats API retrieving by id in
testpmd application: xstats_get() and
xstats_get_names() call with modified parameters.

doc: add description for modified xstats API
Documentation change for modified extended statistics API functions.
The old API only allows retrieval of *all* of the NIC statistics
at once. Given this requires a MMIO read PCI transaction per statistic
it is an inefficient way of retrieving just a few key statistics.
Often a monitoring agent only has an interest in a few key statistics,
and the old API forces wasting CPU time and PCIe bandwidth in retrieving
*all* statistics; even those that the application didn't explicitly
show an interest in.
The new, more flexible API allow retrieval of statistics per ID.
If a PMD wishes, it can be implemented to read just the required
NIC registers. As a result, the monitoring application no longer wastes
PCIe bandwidth and CPU time.

Signed-off-by: Jacek Piasecki <jacekx.piasecki@intel.com>
Signed-off-by: Kuba Kozak <kubax.kozak@intel.com>
Signed-off-by: Tomasz Kulasek <tomaszx.kulasek@intel.com>
Acked-by: Harry van Haaren <harry.van.haaren@intel.com>