Make it possible to start UTM on macOS boot without having to login

[proski]

When UTM is used to run VMs that are part of IT infrastructure, it should start on boot automatically. If the host is restarted e.g. due to power failure or a security update, sysadmins should not be required to log in to every host and start every VM manually.

For UTM to start on boot, it should be run by launchd as a daemon, defined in /Library/LaunchDaemons. That is not currently working.

Reasons not to just use QEMU:

• If a VM was created in UTM, it is less risky to keep it running in UTM rather than switch to QEMU.
• VMs using Apple Virtualization Framework cannot be converted to QEMU images. UTM doesn't currently support the conversion.
• Apple Virtualization Framework may be preferred for some workloads.

I'm aware of previous requests to start UTM on boot (#4179), but in that case the requester settled on starting UTM on login. I also see PRs attempts to control UTM in a headless way (#5081). This issue it about UTM itself starting in a headless way.

[wjk]

I don’t think this will work. I don’t think there is anything stopping the user from dropping a launchd plist into /Library/LaunchDaemons, as long as UTM is configured to run as the user it is normally opened as, but we can’t do that ourselves because it requires root access and sandboxed apps can’t acquire that. For a macOS app to run there needs to be a lot of background services running. None of these services are running until a user logs in. In order to work around this, we would need to have an alternate start path that bypasses NSApplicationMain() or the SwiftUI equivalent and runs the VMs directly. Furthermore, if we do that, then we would have no way to communicate with the daemonized copy of UTM from the UI because it wouldn’t be running in the same user session.

I have no evidence on whether or not SMAppService will work in a sandboxed app, but I believe the answer is no, and I also believe using it might be against App Store policy (although, as with anything in the App Store, there is no way to know for sure until after the feature is already written).