fix(KeepAlive): adapt keepalive for ssr (#3259)

fix #3255

packages/server-renderer/__tests__/render.spec.ts

@@ -8,6 +8,7 @@ import {
   defineComponent,
   createTextVNode,
   createStaticVNode,
+  KeepAlive,
   withCtx
 } from 'vue'
 import { escapeHtml } from '@vue/shared'
@@ -604,6 +605,17 @@ function testRender(type: string, render: typeof renderToString) {
       })
     })
 
+    describe('vnode component', () => {
+      test('KeepAlive', async () => {
+        const MyComp = {
+          render: () => h('p', 'hello')
+        }
+        expect(await render(h(KeepAlive, () => h(MyComp)))).toBe(
+          `<!--[--><p>hello</p><!--]-->`
+        )
+      })
+    })
+
     describe('raw vnode types', () => {
       test('Text', async () => {
         expect(await render(createTextVNode('hello <div>'))).toBe(