Add option to create image with backing image option set

[abbbi]

Now that the new drive backup api is implemented, the created images could already set the backing image of the prior backup (the utility creates the image, not qemu itself anymore)

So during full backup the target image is created:

qemu-img create -f qcow .. FULL-timestamp-imagename.partial

-> Data is pushed to image .partial and if all actions are complete is renamed to FULL-timestamp-imagename.

The next incremental backup could then:

1. find the latest image in the target directory
2. create the new image for the incremental backup and already set the backing image information to the last backup created:

qemu-img crate -f qcow ... -b FULL-timestamp-imagename INC1-timestamp-imagename

-> push incremental data to INC-timestamp-imagename

The next incremental backup

qemu-img crate -f qcow ... -b INC1-timestamp-imagename INC2-timestamp-imagename

and so on.

This obsoletes the rebase functionality as images are already contain all references required for immediate use.

Should be implemented as option, because it might be the case that the last image is not available if it is moved to off-site location:

https://qemu-project.gitlab.io/qemu/interop/bitmaps.html#example-incremental-push-backups-without-backing-files