Commit 000e9504 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