util: storage: Make @backingFormat optional in virStorageFileGetMetadataInternal

Some callers don't need to know the backing format. Make the argument
optional by using a dummy int if NULL is passed.

src/util/virstoragefile.c

@@ -966,9 +966,13 @@ virStorageFileGetMetadataInternal(virStorageSourcePtr meta,
                                   size_t len,
                                   int *backingFormat)
 {
+    int dummy;
     int ret = -1;
     size_t i;
 
+    if (!backingFormat)
+        backingFormat = &dummy;
+
     VIR_DEBUG("path=%s, buf=%p, len=%zu, meta->format=%d",
               meta->path, buf, len, meta->format);
 

src/util/virstoragefile.h

@@ -293,7 +293,7 @@ int virStorageFileGetMetadataInternal(virStorageSourcePtr meta,
                                       char *buf,
                                       size_t len,
                                       int *backingFormat)
-    ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2) ATTRIBUTE_NONNULL(4);
+    ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2);
 
 virStorageSourcePtr virStorageFileGetMetadataFromFD(const char *path,
                                                     int fd,