FVB shall enable --force not only for Luke.

Addressing:
    $ ./fvb -d fedora-19 -n foreman-f19
    Parsing extra arguments
    Image /var/lib/libvirt/images/foreman-f19.local.lan.img exists, use --force Luke!
    $ ./fvb --force -d fedora-19 -n foreman-f19
    getopt: unrecognized option '--force'