[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/AbstractVnetService.java b/incubator/api/src/main/java/org/onosproject/incubator/net/virtual/AbstractVnetService.java
new file mode 100644
index 0000000..2eac0af
--- /dev/null
+++ b/incubator/api/src/main/java/org/onosproject/incubator/net/virtual/AbstractVnetService.java
@@ -0,0 +1,48 @@
+/*
+ * Copyright 2017-present Open Networking Laboratory
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.onosproject.incubator.net.virtual;
+
+import org.onlab.osgi.DefaultServiceDirectory;
+import org.onlab.osgi.ServiceDirectory;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+
+/**
+ * Basis for virtual network service.
+ */
+public abstract class AbstractVnetService
+        implements VnetService {
+
+    private static final String NETWORK_NULL = "Network ID cannot be null";
+
+    protected NetworkId networkId;
+    protected VirtualNetworkService manager;
+    protected ServiceDirectory serviceDirectory;
+
+    public AbstractVnetService(VirtualNetworkService manager,
+                               NetworkId networkId) {
+        checkNotNull(networkId, NETWORK_NULL);
+        this.manager = manager;
+        this.networkId = networkId;
+        this.serviceDirectory = new DefaultServiceDirectory();
+    }
+
+    @Override
+    public NetworkId networkId() {
+        return this.networkId;
+    }
+}