qemu: domain: Parse and format relPath into disk source private data

Register the helpers directly to format and parse the data.

https://bugzilla.redhat.com/show_bug.cgi?id=1523261

src/qemu/qemu_domain.c

@@ -2451,6 +2451,8 @@ virDomainXMLPrivateDataCallbacks virQEMUDriverPrivateDataCallbacks = {
     .chrSourceNew = qemuDomainChrSourcePrivateNew,
     .parse = qemuDomainObjPrivateXMLParse,
     .format = qemuDomainObjPrivateXMLFormat,
+    .storageParse = virStorageSourcePrivateDataParseRelPath,
+    .storageFormat = virStorageSourcePrivateDataFormatRelPath,
 };