Fix to make things serializable

Change-Id: Id6ec282d72065241f8942c2b169311de5710a6b7

@@ -45,4 +45,9 @@ public class MessageSubject {
         MessageSubject that = (MessageSubject) obj;
         return Objects.equals(this.value, that.value);
     }
+
+    // for serializer
+    protected MessageSubject() {
+        this.value = "";
+    }
 }

@@ -35,4 +35,11 @@ public class InternalDeviceEvent {
     public Timestamped<DeviceDescription> deviceDescription() {
         return deviceDescription;
     }
+
+    // for serializer
+    protected InternalDeviceEvent() {
+        this.providerId = null;
+        this.deviceId = null;
+        this.deviceDescription = null;
+    }
 }

@@ -37,4 +37,11 @@ public class InternalPortEvent {
     public Timestamped<List<PortDescription>> portDescriptions() {
         return portDescriptions;
     }
+
+    // for serializer
+    protected InternalPortEvent() {
+        this.providerId = null;
+        this.deviceId = null;
+        this.portDescriptions = null;
+    }
 }

@@ -35,4 +35,11 @@ public class InternalPortStatusEvent {
     public Timestamped<PortDescription> portDescription() {
         return portDescription;
     }
+
+    // for serializer
+    protected InternalPortStatusEvent() {
+        this.providerId = null;
+        this.deviceId = null;
+        this.portDescription = null;
+    }
 }