Don't reuse the return value of virStorageBackendFileSystemIsMounted.
If it's 0, we'd return it even if the mount command failed.

Also, don't report another error if it's -1, since one has already
been reported.

Introduced by 258e06c8.

https://bugzilla.redhat.com/show_bug.cgi?id=981251
(cherry picked from commit 13fde7ce)