conf: Annotate that private data for objects are not copied

Our copy functions format and parse XML thus are not able to copy data.
Annotate the private data pointers that this is happening.

src/conf/domain_conf.h

@@ -2404,6 +2404,8 @@ typedef virDomainXMLPrivateDataCallbacks *virDomainXMLPrivateDataCallbacksPtr;
 struct _virDomainXMLPrivateDataCallbacks {
     virDomainXMLPrivateDataAllocFunc  alloc;
     virDomainXMLPrivateDataFreeFunc   free;
+    /* note that private data for devices are not copied when using
+     * virDomainDefCopy and similar functions */
     virDomainXMLPrivateDataNewFunc    diskNew;
     virDomainXMLPrivateDataNewFunc    hostdevNew;
     virDomainXMLPrivateDataFormatFunc format;