ofp-util: Make "OXM" flow format name include both OpenFlow 1.2 and 1.3.

ofp-util: Make "OXM" flow format name include both OpenFlow 1.2 and 1.3.

To my mind, it makes sense, when a user requests OXM as the flow format,
to allow any protocol based on the OXM flow format to be used.  Until this
commit, however, "OXM" was specifically OpenFlow 1.2, and "OpenFlow13" was
required to request OXM on OpenFlow 1.3.

This doesn't affect the behavior of any released version of Open vSwitch.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Simon Horman <horms@verge.net.au>