g364fb: add VMStateDescription for G364SysBusState

Currently when QEMU attempts to migrate the MIPS magnum machine it crashes due
to a mistake in the g364fb VMStateDescription configuration which expects a
G364SysBusState and not a G364State.

Resolve the issue by adding a new VMStateDescription for G364SysBusState and
embedding the existing vmstate_g364fb VMStateDescription inside it using
VMSTATE_STRUCT.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Fixes: 97a3f6ff ("g364fb: convert to qdev")
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210625163554.14879-3-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>