Use self-explicit definitions instead of magic '512' value.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20200814082841.27000-8-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>