aspeed: Create sd devices only when defaults are enabled

aspeed: Create sd devices only when defaults are enabled

When the -nodefaults option is set, sd devices should not be
automatically created by the machine. Instead they should be defined
on the command line.

Note that it is not currently possible to define which bus an
"sd-card" device is attached to:

  -blockdev node-name=drive0,driver=file,filename=/path/to/file.img \
  -device sd-card,drive=drive0,id=sd0

and the first bus named "sd-bus" will be used.

Reviewed-by: Jamin Lin <jamin_lin@aspeedtech.com>
Acked-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/qemu-devel/20250122070909.1138598-10-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>