i40e/virtchnl: move function to virtchnl

i40e/virtchnl: move function to virtchnl

This moves a function that is needed for the virtchnl interface
from the i40e PF driver over to the virtchnl.h file.

It was manually verified that the function in question is unchanged
except for the function name and function header, which explains
the slight difference in the number of lines removed/added.

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>