Support for FTDI and other complex devices

[beriberikix]

Moving my comment from #7 here with clarification:

There seems to be lack of support for interfacing with various IoT development boards when acting as a standard USB/IP host server. Most boards have either an FTDI, Silicon Labs (ex. CP2102) or similar USB to serial adapter. These all have vendor subclasses, though they are common enough to be in tree.

When I try the host example, I see empty packets in tests when I would expect data. The exact same hardware worked correctly in my tests worked using the native usbip server on Linux. I've also tested the same set of hardware with servers on Windows (via usbipd-win) and on mac (via pyusbip) and had success.

What's required to support these type of devices? Fortunately there's not that many, but mostly trying to understand if I'm going down the right path.