storage: fs: Don't fail volume update if backing store isn't accessible

When the backing store of a volume wasn't accessible while updating the
volume definition the call would fail altogether. In cases where we
currently (incorrectly) treat remote backing stores as local one this
might lead to strange errors.

Ignore the opening errors until we figure out how to track proper volume
metadata.

src/storage/storage_backend.c

@@ -1465,7 +1465,8 @@ virStorageBackendUpdateVolInfo(virStorageVolDefPtr vol,
         (ret = virStorageBackendUpdateVolTargetInfo(vol->target.backingStore,
                                                     updateCapacity,
                                                     withBlockVolFormat,
-                                                    VIR_STORAGE_VOL_OPEN_DEFAULT)) < 0)
+                                                    VIR_STORAGE_VOL_OPEN_DEFAULT |
+                                                    VIR_STORAGE_VOL_OPEN_NOERROR) < 0))
         return ret;
 
     return 0;