exec/cpu: Have cpu_exec_realize() return a boolean

Following the example documented since commit e3fe3988 ("error:
Document Error API usage rules"), have cpu_exec_realizefn()
return a boolean indicating whether an error is set or not.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230918160257.30127-22-philmd@linaro.org>