cloud-hypervisor: broaden schema coverage with fields added in v42.0-v53.0

Diffing Cloud Hypervisor's OpenAPI spec (vmm/src/api/openapi/cloud-hypervisor.yaml) between v41.0 and v53.0 shows a number of new, optional, additive fields that flintlock's hand-written pkg/cloudhypervisor/types.go doesn't model. None of these are required for basic compatibility (flintlock currently launches cloud-hypervisor via CLI args, not the vm.create REST body, and only uses the REST client for read operations like Info/Counters), but adding them would give fuller API coverage for future features.

Candidate fields

• pci_device_id — added to most device configs (DiskConfig, NetConfig, FsConfig, PmemConfig, VdpaConfig, VsockConfig, DeviceConfig, ConsoleConfig/SerialConfig, RtcConfig) alongside the existing pci_segment
• NetConfig: offload_tso, offload_ufo, offload_csum
• DiskConfig: image_type, lock_granularity, backing_files, sparse
• PlatformConfig: iommufd, sev_snp, vfio_p2p_dma, iommu_address_width_bits, SMBIOS system_manufacturer/system_product_name/system_version/system_family/system_sku_number/chassis_asset_tag
• CpusConfig: core_scheduling (CoreSchedulingMode: Vm/Vcpu/Off), nested
• New RtcConfig type (real-time clock device)
• VMConfig: igvm, host_data (payload-related), user_devices, generic-vhost-user arrays

Filed as part of the v53.0 compatibility review. Deliberately scoped out of the minimal v53.0 version-bump PR — pick up only as needed when a feature requires one of these fields.

Compare vmm/src/api/openapi/cloud-hypervisor.yaml across v41.0 and v53.0, then inspect the corresponding definitions in pkg/cloudhypervisor/types.go. Model the listed optional schema fields and new types that fit the compatibility scope, and verify the Go package still builds and its existing REST read operations remain compatible.