Support local disk changes to `DiskCreate`

[sudomateo]

The local storage work in oxidecomputer/omicron#9409 changed the DiskCreate type to have a DiskBackend field. We need to support these changes for our downstream integrations to continue functioning.

[sudomateo]

This might already be done via SSE-129. We'll just need to verify the generated code makes sense.

[sudomateo]

More public API changes coming in oxidecomputer/omicron#9543. We'll need to account for those when merged.