util/uuid: Add UUID_STR_LEN definition

qemu_uuid_unparse() includes a trailing NUL when writing the uuid
string and the buffer size should be UUID_FMT_LEN + 1 bytes. Add a
define for this size and use it where required.

Cc: Fam Zheng <fam@euphon.net>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: "Denis V. Lunev" <den@openvz.org>
Signed-off-by: Cédric Le Goater <clg@redhat.com>

tests/unit/test-uuid.c

@@ -145,7 +145,7 @@ static void test_uuid_unparse(void)
     int i;
 
     for (i = 0; i < ARRAY_SIZE(uuid_test_data); i++) {
-        char out[37];
+        char out[UUID_STR_LEN];
 
         if (!uuid_test_data[i].check_unparse) {
             continue;