-netdev socket uses 100% cpu on Windows host

On Windows hosts, any `-netdev socket` option (tcp listen, tcp connect, udp passing a fd) causes qemu to use 100% cpu. The guest still runs, but only sluggishly.

A simple testcase is:

> qemu-system-i386.exe -netdev socket,listen=:8000,id=n

And, in another command prompt:

> echo foo | nc.exe localhost 8000

Where nc.exe is netcat.

Tested on qemu 3.1 (from https://qemu.weilnetz.de/w64/) and 4.0 (self compiled).