Update the alpha_cpu_realize() signature and hook up to
DeviceClass::realize. Set realized = true in cpu_alpha_init().
qapi/error.h is included through qdev now and no longer needed.
Acked-by: Richard Henderson <rth@twiddle.net>
[AF: Invoke parent's realizefn]
Signed-off-by: Andreas Färber <afaerber@suse.de>