if_ixv: disable RSS configuration on 82599 and X540 VFs

It is reported that those VFs share their RSS configuration with PF and,
thus, they cannot be configured independently.

Also:

• add missing opt_rss.h to if_ixv.c, otherwise RSS kernel option could not be seen
• do not enable IXGBE_FEATURE_RSS on the older VFs
• set flowid / hash type to M_HASHTYPE_NONE or M_HASHTYPE_OPAQUE_HASH (based on what the hardware reports) if IXGBE_FEATURE_RSS is not set

Reviewed by: nobody
MFC after: 4 weeks
Sponsored by: Panzura
Differential Revision: https://reviews.freebsd.org/D21705