storage: util: Properly ignore errors when backing volume is inaccessible

Commit 000e9504559 tried to fix improper bracketing when refreshing disk
volume stats for a backing volume. Unfortunately the condition is still
wrong as in cases as the backing store being inaccessible
storageBackendUpdateVolTargetInfo returns -2 if instructed to ignore
errors. The condition does not take this into account.

Dumping XML of a volume which has inacessible backing store would then
result into:

 # virsh vol-dumpxml http.img --pool default
 error: An error occurred, but the cause is unknown

Properly ignore -2 for backing volumes.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1540022

src/storage/storage_util.c

@@ -1918,7 +1918,7 @@ virStorageBackendUpdateVolInfo(virStorageVolDefPtr vol,
                                                  withBlockVolFormat,
                                                  VIR_STORAGE_VOL_OPEN_DEFAULT |
                                                  VIR_STORAGE_VOL_OPEN_NOERROR,
-                                                 readflags)) < 0)
+                                                 readflags)) == -1)
         return ret;
 
     return 0;