storage: Force setting of disk format type
    
    Commit id '832a9256' adjusted the code to recognize when the default
    type of "unknown" was provided as the format type and to use "dos" if
    found. Since the pool is built with "dos" and it could cause some
    confusion when formatting the XML after building by seeing "unknown"
    in the output, let's just adjust the pool's setting to "dos" so that
    subsequent formats will see the value.