ixgbevf: Support Windows hosts (Hyper-V)

On Hyper-V, the VF/PF communication is a via software mediated path
as opposed to the hardware mailbox. Make the necessary
adjustments to support Hyper-V.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>