docs/interop/firmware.json: add new enum FirmwareFormat

docs/interop/firmware.json: add new enum FirmwareFormat

Only a small subset of all blockdev drivers make sense for firmware
images. Introduce and use a new enum to represent this.

This also reduces the dependency on firmware.json from the global qapi
definitions.

Claim "Since: 3.0" for the new enum, because that's correct for its
members, and the members are what matters in the interface.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240719-qapi-firmware-json-v6-1-c2e3de390b58@linutronix.de>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>