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.
This reverts commit f7ce10db8f76f752cf9d65b420de352e70bdb048.
This should be fixed by doing 0-length reads in qrexec main loop (instead of
real read of fired port number).
