Azure-arm post-processor for disk or vhd

_This issue was originally opened by @mginster as hashicorp/packer#9208. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

Since azure-arm vhd support is going away eventually, will you be investigating a way to still get a managed disk or vhd output? Currently the only output is a managed image and going from that to a vhd is a few unnecessary steps.

Or can we just extend vhd to vhd support? :)

Start with the Azure-arm post-processor and review how it currently produces a managed image. Clarify whether the intended result is managed disk output, VHD output, or VHD-to-VHD support; done means one of those output paths is defined and implemented.

Written by the indexing model from the issue text.