net/core: Add reading VF statistics through the PF netdevice

Add ndo_get_vf_stats where the PF retrieves and fills the VFs traffic
statistics. We encode the VF stats in a nested manner to allow for
future extensions.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>