storageBackendProbeTarget: Don't fail if backing store can't be parsed

storageBackendProbeTarget: Don't fail if backing store can't be parsed

When the backing store of the image can't be parsed
virStorageSourceNewFromBacking returns -1. storageBackendProbeTarget
then also fails which makes the pool refresh fail or even the storage
pool becomes inactive after (re)start of libvirtd.

In situations when we can't access the backing store via network we
just report the backing store string, thus we can do the same thing for
unparsable backing store to prevent the pool from going offline.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>