i40e/i40evf: add VIRTCHNL_VF_OFFLOAD flag

Add virtual channel offload capability to support RX polling mode in the
VF.

Change-ID: Ib643ae2a7506dfc75fc489fc207493fabefa4832
Signed-off-by: Jingjing Wu <jingjing.wu@intel.com>
Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>