Unify format_image methods across cloud providers

[F-X64]

Some cloud providers (e.g. Azure) require more pre-processing of data before the raw image data can be converted into the format we need. This leads to some format_image methods that can't be called with raw cloud provider data.

To make things less confusing, format_image should be implemented equally across cloud providers or, if this can't be achieved, made private to signal that it shouldn't be called on its own.