VM migration

We define two types of flow adjustment:
1. Same switch migration:
(dpid, port_old, mac) -> (dpid, port_new, mac)
(1) Modify flow at dpid towards mac.
2. Different switch migration:
(dpid_old, port_old, mac) ->
          (dpid_new, port_new, mac)
(1) Install/Update a new flow at dpid_new towards mac.
(2) Redirect all flows on dpidsother than dpid_new towards mac.