mkosi: Build a directory image by default

mkosi: Build a directory image by default

Both building and booting a directory image is much faster than
building or booting a disk image so let's default to a directory
image.

In CI, we stick to a disk image to make sure that keeps working as
well.

The only extra dependency this introduces is virtiofsd which is
packaged in all distributions except Debian stable. For users
hacking on systemd on Debian stable, a disk image can be built by
writing the following to mkosi.local.conf:

```
[Output]
Format=disk
```