devserver: add logging around is_staged rpc

BUG=chromium:621317
TEST=None

Change-Id: Ide6a928c74a22d96b67e2d91240d61ca5e4641d5
Reviewed-on: https://chromium-review.googlesource.com/353777
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
diff --git a/devserver.py b/devserver.py
index 1338345..a3511a5 100755
--- a/devserver.py
+++ b/devserver.py
@@ -666,7 +666,9 @@
             artifacts=autotest,test_suites
     """
     dl, factory = _get_downloader_and_factory(kwargs)
-    return str(dl.IsStaged(factory))
+    response = str(dl.IsStaged(factory))
+    _Log('Responding to is_staged %s request with %r', kwargs, response)
+    return response
 
   @cherrypy.expose
   def list_image_dir(self, **kwargs):