Detect that the controller has become unavailable

Currently the Karabo device is not aware whether the controller is still available or not.

What will happen is that any action done on the device will have no effect if the controller has gone offline.

A possible way to detect a disconnection and trigger a state change could be to use the closed flag for the SSH channel:

• https://docs.paramiko.org/en/3.3/api/channel.html#paramiko.channel.Channel.closed