Addition of helper functions in the QEMU scriptlet

[bensmrs]

Hi! I'm making good progress with my black magic operations to get macOS working, thanks to the new QEMU scriptlet. I’m finding myself defining plenty of functions like qom_get, qom_set,… which are critical to me. Would it be interesting to have them directly available to the scriptlets? I know it will only serve 5 to 10 people, but if that sounds good to you, I’ll add them to the Incus codebase.

[stgraber]

Yeah, that should be fine to add a few of the most common ones as exported functions for this scriptlet.