S390/sysinfo: use uuid_is_null instead of opencoding it

And switch to use uuid_t instead of the old uuid_be type.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>