Fix storage format probing

Commit fff74b27 moved the probing into virStorageFileGetMetadataFromBuf
but didn't update the format in volume definition.

https://bugzilla.redhat.com/show_bug.cgi?id=1104908

src/storage/storage_backend_fs.c

@@ -104,6 +104,8 @@ virStorageBackendProbeTarget(virStorageSourcePtr target,
     /* Default to success below this point */
     ret = 0;
 
+    target->format = meta->format;
+
     if (*backingStore &&
         *backingStoreFormat == VIR_STORAGE_FILE_AUTO &&
         virStorageIsFile(*backingStore)) {

src/storage/storage_backend_gluster.c

@@ -298,6 +298,7 @@ virStorageBackendGlusterRefreshVol(virStorageBackendGlusterStatePtr state,
                                                   &vol->backingStore.format)))
         goto cleanup;
 
+    vol->target.format = meta->format;
     if (vol->backingStore.path &&
         vol->backingStore.format < 0)
         vol->backingStore.format = VIR_STORAGE_FILE_RAW;