sflow: Expose ethernet stats via sFlow

sflow: Expose ethernet stats via sFlow

Expose existing netdev stats via sFlow.
Export sFlow ETHERNET structure with available counters.
Map existing stats to counters in the GENERIC INTERFACE
sFlow structure.
Adjust unit test to accommodate these new counters.

Signed-off-by: Robert Wojciechowicz <robertx.wojciechowicz@intel.com>
Acked-by: Neil McKee <neil.mckee@inmon.com>
Acked-by: Ian Stokes <ian.stokes@intel.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>