plugin/ryu: support AgentExtRpcCallback

This patch fixes the following exception.

> 2013-05-14 03:39:53    ERROR [quantum.openstack.common.rpc.amqp] Exception during message handling
> Traceback (most recent call last):
>   File "/opt/stack/quantum/quantum/openstack/common/rpc/amqp.py", line 433, in _process_data
>     **args)
>   File "/opt/stack/quantum/quantum/common/rpc.py", line 44, in dispatch
>     quantum_ctxt, version, method, namespace, **kwargs)
>   File "/opt/stack/quantum/quantum/openstack/common/rpc/dispatcher.py", line 151, in dispatch
>     raise AttributeError("No such RPC function '%s'" % method)
> AttributeError: No such RPC function 'report_state'

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>