pc-dimm: get memory region from ->get_memory_region()

Curretly, the memory region of backed memory is all directly
mapped to guest's address space, however, it will be not true
for nvdimm device if we introduce nvdimm label which only can
be indirectly accessed by ACPI DSM method

Also it improves the comments a bit to reflect this fact

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>
