As Ignition supports KubeVirt, add a custom oem for it and also the
required parts to be able to build an image in .qcow2 format that
is already using internal .qcow2 gzip compression.
Fixes: https://github.com/flatcar/Flatcar/issues/1358
Signed-off-by: Adrian Vladu <avladu@cloudbasesolutions.com>