Revisiting trying to get qemu 2.4.1 to compile, it will take a lot of work. It stopped for me with missing daemon support, which cold be ported from squid for example. Looking at that part of the code is an expectation of ptty support, which also could be worked around then other potential problems needing a skilled OS/2 programmer,
Network working, requires a TAP driver
Support for physical devices, at least CD's and ideally floppies too. Not sure if HD also should be included
USB, our libusb might be good enough
kernel mode driver, kqemu
perhaps stuff like the virtual box additions for clipboard, mouse etc
Sure there is more, then there's making it stable.