Comment 13 for bug 1847105

This behaviour changed due to this commit:

commit c9ec7088c7a3f4cd26bb471f1f243931fff6f4f9
Author: Wim ten Have <email address hidden>
Date: Mon Apr 9 20:14:35 2018 +0200

    storage: extend preallocation flags support for qemu-img

    This patch adds support to qcow2 formatted filesystem object storage by
    instructing qemu-img to build them with preallocation=falloc whenever the
    XML described storage <allocation> matches its <capacity>. For all other
    cases the filesystem stored objects are built with preallocation=metadata.

    Signed-off-by: Wim ten Have <email address hidden>
    Signed-off-by: Michal Privoznik <email address hidden>

This is a semantic change, so should not have been allowed.

It has been 1 & 1/2 years already, but I'm wondering if we should none the less revert it and instead add some other way to declare which preallocation strategy to use.