Add discard_no_unref option for qcow2 images

Add discard_no_unref option for qcow2 images

Qemu 8.1.0 will add discard_no_unref option for qcow2 images.
When this option is enabled (default=false), then it will no longer
unreference clusters when guest does a discard, but it will just free
the blocks (useful for incremental backups for example) and pass the
discard to the lower layer.

This was implemented to avoid fragmentation within the qcow2 image.

Signed-off-by: Jean-Louis Dupond <jean-louis@dupond.be>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>