message: fix ListPhysicalDevice format for device type
diff: @@ -169,7 +169,7 @@ Status PluggableDeviceFactory::ListPhysicalDevices(
  int device_count = platform->VisibleDeviceCount();
  for (int i = 0; i < device_count; ++i) {
  const string device_name =
  - strings::StrCat("/physical_device:", device_type_, i);
  + strings::StrCat("/physical_device:", device_type_, ":", i);
  devices->push_back(device_name);
  }
  