Thus far, the python test code has only ever sent messages of type
commands to the server and processed the corresponding replies. For the
twin-socket feature, the tests will exercise flows where DMA access
commands must be received, processed, and replied to by the client.

This change refactors the message handling python test code to provide
functions to handle server-to-client commands, reusing existing code as
appropriate.

Signed-off-by: Mattias Nissler <[email protected]>