By default, `qemu-img convert` creates qcow2 images usable in qemu 1.1 and
newer. RHEL 8 guest images are meant to be bootable on RHEL 6 though.
Unfortunately, RHEL 6 has qemu 0.12, therefore these images cannot be used
there.
To fix this, we need to use the new qcow2_compat option in qemu assembler
to override the default compat version and make qcow2 images that can be used
in qemu 0.10 and newer.
For this, we need osbuild 28 that isn't yet available in of any of
downstreams, therefore we need to pin it everywhere.
Signed-off-by: Ondřej Budai <ondrej@budai.cz>
