Too many path fault logs

When the VTN Manager can not find the physical packet route,
it records an error log which indicates the path fault.

2014-06-12 09:35:56.930 GMT+09:00 [SwitchEvent Thread] ERROR \
o.o.v.m.internal.cluster.VBridgeImpl - default:vtn_1.bridge_1: \
Path fault: OF|00:00:00:00:00:00:00:06 -> OF|00:00:00:00:00:00:00:03

A path fault log is recorded whenever the VTN Manager fails to determine
a physical route of a packet reported by PACKET_IN.
Path fault log should not be recorded if it is already detected.