[Composefs] Add a warning that only the OCI image format is supported

[chlorodose]

Install with composefs backend with an docker-format image will fail with confusing error error: Installing to disk: Setting up composefs boot: Opening stream '': Creating new splitstream reader: Invalid splitstream content type. Transform image to oci format can fix this.

We should print an clear error message when docker-format image is detected.

GOD KNOWS HOW LONG I BEEN TO DEBUG THIS ERROR!