aspeed/scu: Use qemu_guest_getrandom_nofail

aspeed/scu: Use qemu_guest_getrandom_nofail

The random number is intended for use by the guest.  As such, we should
honor the -seed argument for reproducibility.  Use the *_nofail routine
instead of rolling our own error handling locally.

Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>