storage: return -1 when fs pool can't be mounted
storage: return -1 when fs pool can't be mounted

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 258e06c.

https://bugzilla.redhat.com/show_bug.cgi?id=981251

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