win/vchan: remove outstanding evtchn events on libvchan_wait
win/vchan: remove outstanding evtchn events on libvchan_wait

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.

woju/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin