ixgbe: add VF IPsec offload enable flag

Add a private flag to expressly enable support for VF IPsec offload.
The VF will have to be "trusted" in order to use the hardware offload,
but because of the general concerns of managing VF access, we want to
be sure the user specifically is enabling the feature.

This is likely a candidate for becoming a netdev feature flag.

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