fix(v-model): prevent double v-model update on functional components(#8436)

[wildan2711]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:
Currently, when transforming data.model in createElement function, it does not check if the component is a functional component, causing a double update on v-model. This prevents double update on v-model in functional components by not transforming model when Ctor.options.functional is true.

fix #8436 and eventually vuetifyjs/vuetify#4460

[yyx990803]

Thanks for the PR - unfortunately, this does not really fix the root cause (the double-merging would happen regardless of the wrapper being functional or not). A more complete fix was landed in 7b7164c.