storagefile: Don't access backingStoreRaw directly in FromBackingRelative

For the only usage, the rel == parent->backingStoreRaw, so drop
the direct access
Reviewed-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: NCole Robinson <crobinso@redhat.com>

src/util/virstoragefile.c

@@ -2626,7 +2626,7 @@ virStorageSourceNewFromBackingRelative(virStorageSourcePtr parent,
         return NULL;
 
     /* store relative name */
-    if (VIR_STRDUP(def->relPath, parent->backingStoreRaw) < 0)
+    if (VIR_STRDUP(def->relPath, rel) < 0)
         return NULL;
 
     if (!(dirname = mdir_name(parent->path))) {