vm/windows: Wait for VDE switch to startup.

This could possibly cause flapping whenever qemu is too fast in starting
up. As we are running with the shell's -e flag, the socat check also
ensures that the VDE switch is properly started and causes the whole
build to fail, should it not start up within 20 seconds.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>