Find a usable display, which doesn't have an existing Xvfb file

def _find_display(self):
        """
        Find a usable display, which doesn't have an existing Xvfb file
        """
        self.display_num = 2
        while os.path.isdir(XVFB_PATH % (self.display_num,)):
            self.display_num += 1