Add troubleshooting information for VMs not shutting down properly

When the update is run, we shut down VMs on the compute nodes. In
some cases, these VMs may not shut down properly. While this is not
due to the update process itself, it may result in a confusing
situation for the user. We have documented how to diagnose this
issue and offered suggestions on its resolution.

Change-Id: I418144e027f4be84f7485ab19741e34bd8bb808e