ethdev: new method to retrieve extended statistics

ethdev: new method to retrieve extended statistics

This method can be implemented by a poll mode driver to provide
non-standard statistics (which are not part of the generic statistics
structure). Each statistic is returned in a generic form: "name" and
"value" and can be used to dump PMD-specific statistics in the same way
than ethtool in linux kernel.

If the PMD does not provide the xstats_get and xstats_set functions, the
ethdev API will return the generic statistics in the xstats format
(name, value).

This commit opens the door for a clean-up of the generic statistics
structure, only keeping statistics that are really common to all PMDs
and moving specific ones into the xstats API.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Reviewed-by: David Marchand <david.marchand@6wind.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
[Thomas: fix some comments]
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>