i40e: implement VF stats NDO

Implement the VF stats gathering via the kernel via ndo_get_vf_stats().
The driver will show per-VF stats in the output of the command:
ip -s link show dev <PF>

Testing Hints:
ip -s link show dev eth0
will return non-zero VF stats.
...
   vf 0 MAC 00:55:aa:00:55:aa, spoof checking on, link-state enable, trust off
   RX: bytes  packets  mcast   bcast
   128000     1000     104     104
   TX: bytes  packets
   128000     1000

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>