The spice-server assumed that the client is connected until it gets disconnected
When the client becomes unresponsive, e.g. suspended or network cable is out, the server continue trying communicating with it. Queues (such as driver commands ring) got filled, and the guest display driver got stuck (waiting for something to get free -- not sure we should mention this).
The spice-server now monitors if the spice client is responsive, and disconnect if it's not.
If a spice client is unresponsive, the spice server disconnects it.