cloud-hypervisor: migrate off deprecated legacy SMBIOS platform keys

Nobody has claimed this yet.

As of Cloud Hypervisor v53.0, the legacy `--platform` SMBIOS keys `serial_number` and `uuid` are deprecated in favour of new SMBIOS-structure-scoped keys. The legacy keys still work today (with a warning), and Cloud Hypervisor's API stability guarantee means they won't be removed without at least 2 major releases' notice, but flintlock should migrate before they're dropped.

## Work
- Update `pkg/cloudhypervisor/types.go`'s `PlatformConfig.SerialNumber`/`PlatformConfig.UUID` (or their CLI-arg equivalents, if flintlock sets these via `--platform` args rather than the REST body) to use the new SMBIOS-structure-scoped keys
- Confirm current behaviour still works against the latest CH release before making the change (deprecated fields are additive-safe for now)

Filed as part of the v53.0 compatibility review. Low priority — no forcing function yet, just tracking the deprecation.

Start in pkg/cloudhypervisor/types.go and inspect PlatformConfig.SerialNumber and PlatformConfig.UUID, then determine whether flintlock sends them through the REST body or --platform arguments. Confirm the current behavior against the latest Cloud Hypervisor release and update the keys to the new SMBIOS-structure-scoped forms; done means the deprecated legacy keys are no longer used and behavior still works.

Written by the indexing model from the issue text.