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
```

1 files changed, 17 insertions, 0 deletions

diff --git a/docs/HACKING.md b/docs/HACKING.md
index feec6b4537..bd792092f7 100644
--- a/docs/HACKING.md
+++ b/docs/HACKING.md
@@ -56,6 +56,23 @@ $ mkosi qemu
 Every time you rerun the `mkosi` command a fresh image is built, incorporating
 all current changes you made to the project tree.
 
+By default a directory image is built. This requires `virtiofsd` to be installed
+on the host. To build a disk image instead which does not require `virtiofsd`,
+add the following to `mkosi.local.conf`:
+
+```conf
+[Output]
+Format=disk
+```
+
+To boot in UEFI mode instead of using QEMU's direct kernel boot, add the following
+to `mkosi.local.conf`:
+
+```conf
+[Host]
+QemuFirmware=uefi
+```
+
 Putting this all together, here's a series of commands for preparing a patch
 for systemd: