drivers, ixgbe: export vf statistics

This change retrieves network metrics for virtual functions from the
device and exports them via the iproute2 interface.

The code for retrieving the statistics from the device is taken from the
out-of-tree driver.  The feature was introduced with version 2.0.75.7,
so the diff between this version and the previous version 2.0.72.4 was
used to identify required changes. The export via ethtool is omitted in
favor of using the standard ndo_get_vf_stats interface.

Per-VF statistics can now be printed, for instance, via

  ip --statistics link show dev eth0

Signed-off-by: Maximilian Heyne <mheyne@amazon.de>
Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>