https://help.ubuntu.com/community/KVM/Access
    systemctl enable serial-getty@ttyS0.service
    systemctl start serial-getty@ttyS0.service

    http://0pointer.de/blog/projects/serial-console.html
    man
        kernel-command-line(7)
        agetty(8)
        systemd-getty-generator(8)
            konsole=... kernel param
                ttyS0

    # does not work for standard nixos installer vm
    sudo systemctl enable --now serial-getty@ttyS0

    https://ostechnix.com/how-to-enable-virsh-console-access-for-kvm-guests/
        virsh edit <vmname>
        # nixos gui installer vm already have:
            <serial type='pty'>
                <target type='isa-serial' port='0'>
                    <model name='isa-serial'/>
                </target>
            </serial>
            <console type='pty'>
                <target type='serial' port='0'/>
            </console>

# serial-getty@hvc0.service - enabled (not a ttyS0)