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>

pkgs/build-support/vm/windows/controller/default.nix

@@ -148,6 +148,8 @@ let
     QEMU_VDE_SOCKET="$(pwd)/vde.ctl"
     MONITOR_SOCKET="$(pwd)/monitor"
     ${vde2}/bin/vde_switch -s "$QEMU_VDE_SOCKET" &
+    echo 'alive?' | ${socat}/bin/socat - \
+      UNIX-CONNECT:$QEMU_VDE_SOCKET/ctl,retry=20
   '';
 
   vmExec = if installMode then ''