libnvdimm, namespace: Drop uuid_t implementation detail

There is no need for caller to know how uuid_t type is constructed. Thus,
whenever we use it the implementation details are not needed. Drop it for good.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>