An input parameter to VioGetConfig.
Length specifies the length of the data structure in bytes including Length
itself. The maximum size structure required is determined by issuing VioGetConfig
with Length set to 2. When Length is set to 2 on input, VioGetConfig returns
the maximum size structure required in the Length field on output. When
Length is not equal to 2 on input, the Length field is unmodified on output.