libnvdimm/labels: Add type-guid helpers

In preparation for CXL label support, which does not have the type-guid
concept, wrap the existing users with nsl_set_type_guid, and
nsl_validate_type_guid. Recall that the type-guid is a value in the ACPI
NFIT table to indicate how the memory range is used / should be
presented to upper layers.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/162982116208.1124374.13938280892226800953.stgit@dwillia2-desk3.amr.corp.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>