nx-match: Support variable length header lookup.

nx-match: Support variable length header lookup.

Currently we treat the entire NXM/OXM header, including length,
as an ID to define a field. However, this does not allow for
multiple lengths of a particular field.

If a field has been marked as variable, we should ignore the length
when looking up the field and only use the class and field. We
continue to use the length for non-variable fields to ensure that
we don't accept something that can never match.

Signed-off-by: Jesse Gross <jesse@nicira.com>