i40e: implement virtual device interface

While not part of this patch series, an i40evf driver is on its
way, and uses these files to communicate to the PF driver.

This patch contains the header and implementation files for the
PF to VF interface.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
CC: PJ Waskiewicz <peter.p.waskiewicz.jr@intel.com>
CC: e1000-devel@lists.sourceforge.net
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>