Add APIs to get information about data flows.

"Data flow" means a sequence of flow entries which establishes
end-to-end packet flow. APIs added by this patch provide the way to
derive data flow information from the VTN Manager.

  * Condition to select packets.
  * Flow actions to be applied to packets.
  * Packet route in virtual and physical network.
  * Statistics information.

Change-Id: I9d68eadb68fa0865d76c4c772925672327316d91
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>