Windows evtchn driver can double fire on one event (details in
xenpci/evtchn_device_interface.c:EvtChn_EvtIoWrite). Less intrusive way of
fixing it is just remove all events from queue on libvchan_wait (to prevent
queue fill).
This won't hurt because we are interested in events only sent
after xc_evtchn_unmask call which is one line below.