storage_util: fix qemu-img sparse allocation

storage_util: fix qemu-img sparse allocation

Commit <c9ec7088c7a3f4cd26bb471f1f243931fff6f4f9> introduced a support
to fully allocate qcow2 images when <allocation> matches <capacity> but
it doesn't work as expected.

The issue is that info.size_arg is in KB but the info.allocation
introduced by the mentioned commit is in B. This results in using
"preallocation=falloc," in cases where "preallocation=metadata," should
be used.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>