i40e/i40evf: create and use new unified header file

This moves a header for i40evf to include/linux/avf/virtchnl.h.
The directory name AVF is an acronym for the Intel(R) Adaptive
Virtual Function.

This first step creates the new file, which is a rename of
drivers/net/ethernet/intel/i40evf/i40e_virtchnl.h to
include/linux/avf/virtchnl.h, and should show up in git
as a rename when using git log --follow.

To keep things building after the move, the changes to the i40evf
driver are made to point to the new include file location.

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>