nvdimm: make persistence option symbolic

Replace the "nvdimm-cap" option which took numeric arguments such as "2"
with a more user friendly "nvdimm-persistence" option which takes symbolic
arguments "cpu" or "mem-ctrl".

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Suggested-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>