Construct GraphDescription with online/active Device/Link only

Change-Id: I9312c0c8ae190bf0200bd040823b6f7a3e7a15e1
diff --git a/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java b/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java
index 95723b3..b00b993 100644
--- a/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java
+++ b/core/net/src/main/java/org/onlab/onos/net/device/impl/DeviceManager.java
@@ -149,6 +149,11 @@
     }
 
     @Override
+    public Iterable<Device> getAvailableDevices() {
+        return store.getAvailableDevices();
+    }
+
+    @Override
     public Device getDevice(DeviceId deviceId) {
         checkNotNull(deviceId, DEVICE_ID_NULL);
         return store.getDevice(deviceId);