Enable root autologin on serial console
See QubesOS/qubes-issues#5799.
Use an option to agetty:
https://wiki.archlinux.org/index.php/Getty#Automatic_login_to_virtual_console
The --login-pause causes agetty to wait for Enter key. This is
important, because otherwise the root session prevents systemd from
shutting down, and probably causes other side effect.