hw/vmapple/bdif: Introduce vmapple backdoor interface

hw/vmapple/bdif: Introduce vmapple backdoor interface

The VMApple machine exposes AUX and ROOT block devices (as well as USB OTG
emulation) via virtio-pci as well as a special, simple backdoor platform
device.

This patch implements this backdoor platform device to the best of my
understanding. I left out any USB OTG parts; they're only needed for
guest recovery and I don't understand the protocol yet.

Signed-off-by: Alexander Graf <graf@amazon.com>
Signed-off-by: Phil Dennis-Jordan <phil@philjordan.eu>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Tested-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-ID: <20241223221645.29911-11-phil@philjordan.eu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>