pc-dimm: introduce get_vmstate_memory_region callback

This callback returns the MemoryRegion that is the memory of dimm should
be kept during live migration

nvdimm device is different with pc-dimm as its memory includes not only
the MemoryRegion directly mapping to guest's address space but also the
memory used as label data

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>
