ToUserDevice nits

These are some small nits in ToUserDevice.

There is a larger problem, that the config might change during a ToUserDevice::dev_read or FromUserDevice::dev_write and cause elem to become invalid. I believe that this code might need to hook lock_config + unlock_config from clickfs but I'm not sure if that's the best solution. I'm open to suggestions.

I am not sure whether we should add a router block + unblock during a read or a write, but that's a different issue.