hw/uefi: add var-service-json.c + qapi for NV vars.

hw/uefi: add var-service-json.c + qapi for NV vars.

Define qapi schema for the uefi variable store state.

Use it and the generated visitor helper functions to store persistent
(EFI_VARIABLE_NON_VOLATILE) variables in JSON format on disk.

Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-ID: <20250225163031.1409078-15-kraxel@redhat.com>
[ incremental fix squashed in ]
Message-ID: <pji24p6oag7cn2rovus7rquo7q2c6tokuquobfro2sqorky7vu@tk7cxud6jw7f>