mkosi: Enable --qemu-headless option for all distros

mkosi: Enable --qemu-headless option for all distros

--qemu-headless configures the generated image and mkosi's qemu
command to connect to the VM via the serial port. This allows
spawning a qemu VM within the user's terminal instead of spawning
a graphical GTK GUI. --qemu-headless sets TERM, COLUMNS and LINES
in serial-getty@ttyS0.service in the container which makes the
terminal in the VM behave almost equivalent to the one on the host.

This change makes testing changes to systemd using mkosi + QEMU a
lot easier compared to before as commands can be executed in the VM
from the comfort of one's terminal compared to the Linux console
available when running via the GTK GUI.