Make binding InstanceIdentifier serializable

typedef network-topology-ref

{ type instance-identifier; ext:context-reference "network-topology-context"; }

Bug: Class org.opendaylight.yang.gen.v1.urn.opendaylight.params.xml.ns.yang.network.topology.rev140113.NetworkTopologyRef defines non-transient non-serializable instance field _value
This Serializable class defines a non-primitive instance field which is neither transient, Serializable, or java.lang.Object, and does not appear to implement the Externalizable interface or the readObject() and writeObject() methods. Objects of this class will not be deserialized correctly if a non-Serializable object is stored in this field.