'Summon a xen object'

def xen(self, base_tree, results_dir='', tmp_dir='', leave=False, kjob=None):

    (results_dir, tmp_dir) = self.setup_dirs(results_dir, tmp_dir)
    build_dir = 'xen'
    return xen.xen(self, base_tree, results_dir, tmp_dir, build_dir, leave, kjob)
