[ONOS-5936] (vCore) Virtual FlowObjective Manager and Store

Changes
1. FlowObjective manager for virtual network is added
2. VirtualFlowObjective store is added
3. SimpleVirtualFlowObjectiveStore is implementation
4. Unit tests are added

Change-Id: I18ff1d440d1f85ca96fff36a33a8b67566031e2c
diff --git a/incubator/store/src/main/java/org/onosproject/incubator/store/virtual/impl/SimpleVirtualFlowRuleStore.java b/incubator/store/src/main/java/org/onosproject/incubator/store/virtual/impl/SimpleVirtualFlowRuleStore.java
index 5f71b57..0952574 100644
--- a/incubator/store/src/main/java/org/onosproject/incubator/store/virtual/impl/SimpleVirtualFlowRuleStore.java
+++ b/incubator/store/src/main/java/org/onosproject/incubator/store/virtual/impl/SimpleVirtualFlowRuleStore.java
@@ -151,8 +151,12 @@
 
     @Override
     public int getFlowRuleCount(NetworkId networkId) {
-
         int sum = 0;
+
+        if (flowEntries.get(networkId) == null) {
+            return 0;
+        }
+
         for (ConcurrentMap<FlowId, List<StoredFlowEntry>> ft :
                 flowEntries.get(networkId).values()) {
             for (List<StoredFlowEntry> fes : ft.values()) {