Raw format is fine to use on systems that have reflinks for example.

I've been investigating why a seemingly innocuous change
(changing compression on OSBuild generated qemu qcow2) would
cause disk images to not boot [1]. I think I have found the issue.

I was first trying to make sure 100% that the files got written
out over the virtiofs mount before the VM got shutdown so I decided
to add a `umount $workdir` to the process. But this ended up with
a `umount: /srv/: target is busy.` error.

When the supermin VM gets run we `cd "${workdir}"` at the end of
supermin-init-prelude.sh. This has the effect of causing all
spawned processes (including PID1/init) to have a cwd of /srv/.

```
bash-5.2# lsof /srv
COMMAND   PID USER   FD   TYPE DEVICE SIZE/OFF     NODE NAME
init        1 root  cwd    DIR   0,26     4096 10485829 /srv
kthreadd    2 root  cwd    DIR   0,26     4096 10485829 /srv
pool_work   3 root  cwd    DIR   0,26     4096 10485829 /srv
kworker/R   4 root  cwd    DIR   0,26     4096 10485829 /srv
...
...
```

Which means it's unlikely that the virtiofs mount ever gets cleanly
unmounted. Let's rework things here so that actual work gets spawned
in a subshell to prevent `init` from having a cwd on the virtiofs mount.

We also add in an `umount` of the cache qcow2 (if exists) and the virtiofs
mount to strengthen our chances of a clean unmount.

[1] coreos#3728

We previously did this in a different way (2a8d1e6) but then
had to revert it (39fdd61) because it caused images to not boot [1].
The root cause appears to have been the virtiofs mount not
being unmounted cleanly from the supermin VM and that is now
fixed so let's switch back to not compressing since we rely on
our outer compression [2].

[1] coreos#3728
[2] coreos/fedora-coreos-tracker#1653 (comment)

Just in case this is what is causing issues with file consistency
when copying out of the supermin VM.

Still trying to figure out data corruption from
coreos#3728