mkosi: turn off qemu headless mode

mkosi: turn off qemu headless mode

We ship the mkosi files to make sure we can test our own code. A good
chunk of our code (and the main reason to use qemu rather than nspawn)
is the EFI code, i.e. in sd-boot and the EFI stub. Hence it's bad idea
to use qemu headless mode, since that means we bypass all that.

Let's hence toggle the defaults here, but keep the line in place, to
make it easy to switch back if someone wants the speed, rather than the
testing.