virt-install output is missing preseed argument in --extra-args

Trying to work on #1, I used the virt-install output from https://github.com/rwmjones/guestfs-tools/blob/master/builder/templates/ubuntu-20.04.virt-install-cmd. It is not starting in automated mode because it's missing the `ks=/preseed.cfg` in the `--extra-args` option.
