metaflow: Allow fields to be marked as variable length.

metaflow: Allow fields to be marked as variable length.

Until now, all fields that OVS can match against have been fixed
size (variable length headers can be skipped during parsing but
the match is fixed). However, Geneve options can vary in size
so we must not require the size of these fields to be known
at compile time.

This allows data types to be annotated with not only their size
but whether the field can be smaller than that. The following
patches will change OpenFlow parsing based on that.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>