Adding multi-instance support for flow stats.

Change-Id: I428c5a7cb58f4f9773a125fc94fb368ed846cb0d
diff --git a/core/api/src/main/java/org/onosproject/net/device/DefaultPortStatistics.java b/core/api/src/main/java/org/onosproject/net/device/DefaultPortStatistics.java
index 0382833..540a945 100644
--- a/core/api/src/main/java/org/onosproject/net/device/DefaultPortStatistics.java
+++ b/core/api/src/main/java/org/onosproject/net/device/DefaultPortStatistics.java
@@ -61,6 +61,22 @@
         this.durationNano = durationNano;
     }
 
+    // Constructor for serializer
+    private DefaultPortStatistics() {
+        this.deviceId = null;
+        this.port = 0;
+        this.packetsReceived = 0;
+        this.packetsSent = 0;
+        this.bytesReceived = 0;
+        this.bytesSent = 0;
+        this.packetsRxDropped = 0;
+        this.packetsTxDropped = 0;
+        this.packetsRxErrors = 0;
+        this.packetsTxErrors = 0;
+        this.durationSec = 0;
+        this.durationNano = 0;
+    }
+
     /**
      * Creates a builder for DefaultPortStatistics object.
      *