i40e/i40evf: add handling of writeback descriptor

If the writeback descriptor buffer was previously created, this gives it
to the AQ command request to be used to save the results.

Change-ID: I8c8a1af81e6ebed6d0a15ed31697fe1a6c4e3708
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>