[xharness] Make sure to mark device listing as completed even if it fails. (#2419)

tests/xharness/Simulators.cs

@@ -536,10 +536,9 @@ namespace xharness
 								continue;
 							connected_devices.Add (d);
 						}
-
-						connected_devices.SetCompleted ();
 					}
 				} finally {
+					connected_devices.SetCompleted ();
 					File.Delete (tmpfile);
 				}
 			});