ethdev: update extending stats strings

ethdev: update extending stats strings

Update the strings used for presenting stats to adhere
to the scheme previously presented. Updated xstats_get()
function to handle Q information only if xstats() is not
implemented in the PMD, providing the PMD with the needed
flexibility to expose its extended Q stats.

Add extended statistic section to the programmers
guide, poll mode driver section. This section describes
how the strings stats are formatted, and how the client
code can use this to gather information about the stat.

Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Maryam Tahhan <maryam.tahhan@intel.com>