Support "ephermeal" images

I would like to propose a feature enhancement for UTM that would greatly benefit users who require a more flexible approach to virtual machine management, similar to the functionality offered by VirtualBox.

The proposed feature involves the ability to return a virtual machine (VM) to a predefined state upon closure, effectively discarding any changes made during the session. This could be implemented as an option in the preferences menu, labeled something akin to "Do Not Save Changes."

With this feature, users would be able to start the VM normally, with the hard disk appearing read-write to the guest. However, upon shutting down the VM, all modifications to the hard disk would be automatically discarded, allowing for a clean slate for the next session. This functionality would be particularly useful for testing and development environments where transient changes are common and do not need to be retained.

I am curious to know if this capability can currently be achieved using existing QEMU command-line options. If so, I would appreciate any guidance on how to implement this feature with the current version of UTM.