nvdimm: implement NVDIMM device abstract

Introduce "nvdimm" device which is based on pc-dimm device type

Currently, nothing is specific for nvdimm but hotplug is disabled

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
