macfb: add VMStateDescription for MacfbNubusState and MacfbSysBusState

macfb: add VMStateDescription for MacfbNubusState and MacfbSysBusState

Currently when QEMU tries to migrate the macfb framebuffer it crashes randomly
because the opaque provided by the DeviceClass vmsd property for both devices
is set to MacfbState rather than MacfbNubusState or MacfbSysBusState as
appropriate.

Resolve the issue by adding new VMStateDescriptions for MacfbNubusState and
MacfbSysBusState which embed the existing vmstate_macfb VMStateDescription
within them using VMSTATE_STRUCT.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20220305155530.9265-2-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>