Azure-arm post-processor for disk or vhd

[ghost]

This issue was originally opened by @mginster as hashicorp/packer#9208. It was migrated here as a result of the Packer plugin split. The original body of the issue is below.

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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? :)