storage: Determine the local storage type right away
    
    When walking the backing chain we previously set the storage type to
    _FILE and let the virStorageFileGetMetadataFromFDInternal update it to
    the correct type later on.
    
    This patch moves the actual storage type determination to the place
    where we parse the backing store name so that the code can later be
    switched to use virStorageFileReadHeader() directly.