storage: Fix the use-after-free memory bug

Introduced by commit e0139e30. virStorageVolDefFree free'ed the
pointers that are still used by the added volume object, this changes
it back to VIR_FREE.

src/storage/storage_driver.c

@@ -1618,7 +1618,7 @@ storageVolCreateXML(virStoragePoolPtr obj,
 cleanup:
     virObjectUnref(volobj);
     virStorageVolDefFree(voldef);
-    virStorageVolDefFree(buildvoldef);
+    VIR_FREE(buildvoldef);
     if (pool)
         virStoragePoolObjUnlock(pool);
     return ret;