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
(cherry picked from commit c110cdb2)

Conflicts:
	src/storage/storage_backend_fs.c
      I didn't backport the refactors - febcb89c and effbb11e

src/storage/storage_backend_fs.c

@@ -100,6 +100,7 @@ virStorageBackendProbeTarget(virStorageSourcePtr target,
 
         if (VIR_STRDUP(*backingStore, meta->backingStoreRaw) < 0)
             goto error;
+        target->format = meta->format;
     }
 
     VIR_FORCE_CLOSE(fd);

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;