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>

hw/core/qdev-properties-system.c

@@ -1114,7 +1114,7 @@ static void get_uuid(Object *obj, Visitor *v, const char *name, void *opaque,
 {
     Property *prop = opaque;
     QemuUUID *uuid = object_field_prop_ptr(obj, prop);
-    char buffer[UUID_FMT_LEN + 1];
+    char buffer[UUID_STR_LEN];
     char *p = buffer;
 
     qemu_uuid_unparse(uuid, buffer);