domtop: Turn parse_argv into void
    
    Currently, the function follows the usual pattern used in our code:
    
      int ret = -1;
      ...
      ret = 0;
     cleanup:
      return ret;
    
    However, the function always call exit() on error, so the cleanup
    label is never jumped onto. Therefore, it doesn't make any sense to
    have the parse_argv function return an integer value, if it
    effectively can return only value of zero.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>