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: NXiao Guangrong <guangrong.xiao@linux.intel.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/mem/pc-dimm.c

@@ -354,8 +354,9 @@ static void pc_dimm_get_size(Object *obj, Visitor *v, const char *name,
     int64_t value;
     MemoryRegion *mr;
     PCDIMMDevice *dimm = PC_DIMM(obj);
+    PCDIMMDeviceClass *ddc = PC_DIMM_GET_CLASS(obj);
 
-    mr = host_memory_backend_get_memory(dimm->hostmem, errp);
+    mr = ddc->get_memory_region(dimm);
     value = memory_region_size(mr);
 
     visit_type_int(v, name, &value, errp);

include/hw/mem/pc-dimm.h

@@ -58,7 +58,8 @@ typedef struct PCDIMMDevice {
 
 /**
  * PCDIMMDeviceClass:
- * @get_memory_region: returns #MemoryRegion associated with @dimm
+ * @get_memory_region: returns #MemoryRegion associated with @dimm which
+ * is directly mapped into the physical address space of guest
  */
 typedef struct PCDIMMDeviceClass {
     /* private */