storage: fs: Drop-in replace use of virStorageFileGetMetadataFromBuf
    
    Use virStorageFileGetMetadataFromFD instead in
    virStorageBackendProbeTarget as it now returns all required data and the
    storage file is already open in a filedescriptor.
    
    Also fix improper error code being returned when virFileReadHeaderFD
    would fail as virStorageBackendUpdateVolTargetInfoFD would set the
    return code to 0.