virtchnl: finish conversion to virtchnl interface

This patch implements the complete version of the virtchnl.h file
with final renames, and fixes the related code in i40e and i40evf.

It also expands comments, and adds details on the usage of
certain fields.

In addition, due to the changes a couple of casts are needed
to prevent errors found by sparse after renaming some fields.

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>