pc-dimm: introduce realize callback

nvdimm needs to  check if the backend memory is large enough to contain
label data and init its memory region when the device is realized, so
introduce realize callback which is called after common dimm has been
realize

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>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
