After upgrading my host system from Ubuntu 16.04 to 18.04.1, I have noticed occasional complete system hangs. The guest OS is Windows 7 64-bit.
The symptoms are that the mouse cursor will still move (sometimes only in the guest), but clicking or selecting anything (on the host or the guest) is ignored. The only thing I have found that remedies the situation is to CTRL-ALT-DEL repeatedly until the guest is shutdown. The host system then becomes responsive again. Restarting the guest restores the guest machine state and everything works properly again.
This happens somewhat randomly.