qemuBlockStorageSourceGetEffectiveNodename: Prepare for missing 'format' driver

Return the effective storage nodename if the format layer is not
present.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

src/qemu/qemu_block.c

@@ -97,7 +97,10 @@ qemuBlockStorageSourceSetFormatNodename(virStorageSource *src,
 const char *
 qemuBlockStorageSourceGetEffectiveNodename(virStorageSource *src)
 {
-    return src->nodenameformat;
+    if (src->nodenameformat)
+        return src->nodenameformat;
+
+    return qemuBlockStorageSourceGetEffectiveStorageNodename(src);
 }