libvirt_lxc: Claim success for --help
    
    So far, if libvirt_lxc binary (usually to be found under
    /usr/libexec/) is run with --help, due to a missing line
    and our usual functions pattern, an 'uknown' error is returned.
    Yeah, the help is printed out, but we should not claim error.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>