Automatic block overlay creation with freeing the write lock

[Rafael Pizarro Solar]

Dear QEMU maintainers,

For reference, I am using QEMU 8.0.0, and running with softmmu-aarch64.

I have been trying implement a feature that directly creates an overlay with the goal of booting multiple instances from the same base image.

Creating a backing file with `qemu-img` does work, but I want to automatize the process by using creating an overlay with the 'qmp_blockdev_snapshot_sync' function or similar.

So far my approach has been to call `qmp_blockdev_snapshot_sync` during startup phase in the `void qmp_x_exit_preconfig(Error **errp)`  stage.

My expected behavior would be that now that there's an overlay, the "write" lock would be released, allowing for a second image to boot, but it fails as the overlay still hasn't released "write" lock for the backing file.

Thank you for your time,  best regards
Rafael