packet lib: provide a default impl of __len__

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>

ryu/lib/packet/packet_base.py

@@ -40,6 +40,9 @@ class PacketBase(object):
     def __init__(self):
         super(PacketBase, self).__init__()
 
+    def __len__(self):
+        return self._MIN_LEN
+
     @property
     def protocol_name(self):
         return self.__class__.__name__