snapshots: expose QEMU full-VM snapshots

[naveenrajm7]

Summary

• Expose named full-VM snapshot create, list, restore, and delete operations through AppleScript and utmctl for QEMU VMs.
• Use QEMU image metadata as the source of truth while reserving and hiding the internal suspend snapshot.
• Report snapshots as unsupported for Apple Virtualization VMs until that backend can preserve equivalent VM state reliably.

Related to #5484.

Testing

Tested by a human on Mac mini (Apple M2, 16 GB), macOS 26.6.2 (25G83).

• Completed a full macOS archive and DMG build.
• Exercised the guest-visible lifecycle on a Debian 12 Vagrant QEMU VM over SSH: created and hashed a baseline file, created a snapshot, deleted the baseline and created a post-snapshot file, stopped and restored the VM, then verified the baseline returned with the same hash and the post-snapshot file was absent.
• Verified snapshot operations through both utmctl and AppleScript, including deletion and an empty final snapshot list.
• Verified that Apple Virtualization VMs report snapshots as unsupported.

The author acknowledges that this change has been tested and/or reviewed by a human in accordance with the UTM AI contribution guidelines.