ovdk_datapath_messages: Introducing ovdk_datapath_messages

Create h file for message structures to be passed to and from
vswitchd. This will enable sharing rather than duplication
of data structures.

Change-Id: I9d9e60fb77aa010e52f4fff96ca1f5e883937a9a
Reviewed-by: Eimear Morrissey <eimear.morrissey@intel.com>
Reviewed-by: Mark B. Kavanagh <mark.b.kavanagh@intel.com>
Signed-off-by: Mark D. Gray <mark.d.gray@intel.com>