shill: Create and register a Cellular device for each ModemManager.Modem.

BUG=chromium-os:17818
TEST=unit tests

Change-Id: Ic35adf35c8021f4c9689e72ddd03776948d036c1
Reviewed-on: http://gerrit.chromium.org/gerrit/4711
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Darin Petkov <petkov@chromium.org>
diff --git a/device.h b/device.h
index caa49aa..28da466 100644
--- a/device.h
+++ b/device.h
@@ -62,6 +62,9 @@
 
   std::string GetRpcIdentifier();
 
+  const std::string &link_name() const { return link_name_; }
+  int interface_index() const { return interface_index_; }
+
   const std::string &FriendlyName() const;
 
   // Returns a string that is guaranteed to uniquely identify this Device
@@ -101,7 +104,7 @@
   PropertyStore store_;
 
   std::vector<ServiceRefPtr> services_;
-  int interface_index_;
+  const int interface_index_;
   bool running_;
   const std::string link_name_;
   const std::string unique_id_;