Construct GraphDescription with online/active Device/Link only

Change-Id: I9312c0c8ae190bf0200bd040823b6f7a3e7a15e1
diff --git a/core/api/src/main/java/org/onlab/onos/net/device/DeviceService.java b/core/api/src/main/java/org/onlab/onos/net/device/DeviceService.java
index d35b1af..7d2a0dd 100644
--- a/core/api/src/main/java/org/onlab/onos/net/device/DeviceService.java
+++ b/core/api/src/main/java/org/onlab/onos/net/device/DeviceService.java
@@ -44,6 +44,14 @@
     Iterable<Device> getDevices();
 
     /**
+     * Returns an iterable collection of all devices
+     * currently available to the system.
+     *
+     * @return device collection
+     */
+    Iterable<Device> getAvailableDevices();
+
+    /**
      * Returns the device with the specified identifier.
      *
      * @param deviceId device identifier