Fix sto_vol tag when no volume is present

cloudcontrol/node/hypervisor/__init__.py

@@ -103,7 +103,8 @@ class Handler(HostHandler):
                 Tag('sto%s_used' % name,
                     lambda sto: sto.capacity - sto.available, 5, 5, storage),
                 Tag('sto%s_type' % name, lambda sto: sto.type, 5, 5, storage),
-                Tag('sto%s_vol' % name, lambda sto: ' '.join(sto.volumes),
+                Tag('sto%s_vol' % name,
+                    lambda sto: ' '.join(sto.volumes) if sto.volumes else None,
                     5, 5, storage),
             ))