Fix: to register eventlisteners for virtualnetwork

Changes.
1. Virtual network service inherite listener service

Change-Id: Ie2911916b2912e3c8cbd2069e9563e1f5e2c837f
diff --git a/incubator/api/src/main/java/org/onosproject/incubator/net/virtual/VirtualNetworkService.java b/incubator/api/src/main/java/org/onosproject/incubator/net/virtual/VirtualNetworkService.java
index 21f721c..07775dd 100644
--- a/incubator/api/src/main/java/org/onosproject/incubator/net/virtual/VirtualNetworkService.java
+++ b/incubator/api/src/main/java/org/onosproject/incubator/net/virtual/VirtualNetworkService.java
@@ -18,6 +18,7 @@
 import com.google.common.annotations.Beta;
 import org.onlab.osgi.ServiceDirectory;
 import org.onosproject.core.ApplicationId;
+import org.onosproject.event.ListenerService;
 import org.onosproject.net.DeviceId;
 
 import java.util.Set;
@@ -26,7 +27,8 @@
  * Service for querying virtual network inventory.
  */
 @Beta
-public interface VirtualNetworkService {
+public interface VirtualNetworkService
+        extends ListenerService<VirtualNetworkEvent, VirtualNetworkListener> {
 
     /**
      * The topic used for obtaining globally unique ids.