util/uuid: Define UUID_STR_LEN from UUID_NONE string

Cc: Fam Zheng <fam@euphon.net>
Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Cédric Le Goater <clg@redhat.com>

include/qemu/uuid.h

@@ -78,9 +78,10 @@ typedef struct {
                  "%02hhx%02hhx-" \
                  "%02hhx%02hhx%02hhx%02hhx%02hhx%02hhx"
 
-#define UUID_STR_LEN (36 + 1)
-
 #define UUID_NONE "00000000-0000-0000-0000-000000000000"
+QEMU_BUILD_BUG_ON(sizeof(UUID_NONE) - 1 != 36);
+
+#define UUID_STR_LEN sizeof(UUID_NONE)
 
 void qemu_uuid_generate(QemuUUID *out);