memory: Have memory_region_init_ram() handler return a boolean

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

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Message-Id: <20231120213301.24349-7-philmd@linaro.org>