[ONOS-5833] (vCore) Refactor vnet services
to enable event delivery mechanism

Changes
1. Abstact Vnet Listener Manager is added
2. Refactor Vnet services to use it

Change-Id: I178342bfc882c0739f216960358a281903e1385a
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 530c67a..2e852e8 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
@@ -16,6 +16,7 @@
 package org.onosproject.incubator.net.virtual;
 
 import com.google.common.annotations.Beta;
+import org.onlab.osgi.ServiceDirectory;
 import org.onosproject.net.DeviceId;
 
 import java.util.Set;
@@ -105,4 +106,10 @@
      */
     <T> T get(NetworkId networkId, Class<T> serviceClass);
 
+    /**
+     * Returns service directory.
+     *
+     * @return a service directory
+     */
+    ServiceDirectory getServiceDirectory();
 }