igbvf: add new driver to support 82576 virtual functions

This adds an igbvf driver to handle virtual functions provided by the
igb driver when SR-IOV has been enabled.  A virtual function is a
lightweight pci-e function that supports a single queue and shares
resources with the 82576 physical function contained within the igb
driver.

To spawn virtual functions from the igb driver all that is needed is to
enable CONFIG_PCI_IOV and have an 82576 Ethernet adapter on a system that
supports SR-IOV in the BIOS.  The virtual functions will appear after the
interface is loaded.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>