genChildren (el): string {
    if (!el || !el.children || !el.children.length) {
      return ''
    }
    return el.children.map(child => this.genElement(child)).join('')
  }
use "_c" to passing the actual vdom host component instance id to slot template
     because the vdom is actually stored in the component's _vnodes
     event hanlders searching depends on this id