libnvdimm/nfit_test: adding support for unit testing enable LSS status

Adding support code to simulate the enabling of LSS status in support of
the Intel DSM v1.6 Function Index 10: Enable Latch System Shutdown Status.
This is only for testing of libndctl support for LSS enable. The actual
functionality requires a reboot and therefore is not simulated. The enable
value is not recorded in nfit_test since there's no DSM to actually query
the current status of the LSS enable.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>