nvdimm: support nvdimm label

Introduce a parameter, 'label-size', which is the size of nvdimm label
data area which is reserved at the end of backend memory. It is required
at least 128k

Two callbacks, read_label_data() and write_label_data(), are used to
operate the label area

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