Fix incorrect exception on os-migrateLive

If the server state is conflict to live-migrate, it raises
400(HTTPBadRequest)exception.We should raise HTTPConflict instead of
HTTPBadRequest.

Change-Id: I40ccb0f268716da20ae4b3de9d295a41a2dba18d
Closes-Bug: #1255001
Merge "Fix incorrect exception on os-migrateLive"