ixgbevf: add ethtool private flag for legacy Rx

Introduce legacy-rx private flag that will allow switching between the
old and new (build_skb based) Rx code paths. The implementation is the
same as in commit e0891298


("igb: Add support for ethtool private flag to allow use of legacy Rx")

This provides a means of validating the legacy Rx path in the event that
we are forced to fall back.  At some point in the future when we are
convinced we don't need it anymore we might be able to drop the legacy-rx
flag.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>