New Python QMP library races on NetBSD

The new python asyncio QMP library still races on netBSD and potentially elsewhere. Even having modified the library to bind early still doesn't save us from the race, because we are not yet listening on the port prior to launching the QEMU process.

Fixing this requires a redesign of the async QMP library API to allow a server to be started from the sync bridge prior to the subprocess call to launch QEMU. I'm working on these changes now.

In the meantime, you might be able to use the env var QEMU_PYTHON_LEGACY_QMP=1 to force the old library which should not have this problem, but it might require some routing to get it to be used in VM tests.