ndtest: Add nvdimm control functions

Add functions to support ND_CMD_GET_CONFIG_SIZE, ND_CMD_SET_CONFIG_DATA and
ND_CMD_GET_CONFIG_DATA.

Signed-off-by: Santosh Sivaraj <santosh@fossix.org>
Link: https://lore.kernel.org/r/20201222042240.2983755-7-santosh@fossix.org


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