epoll: Fix clearing evt.events in EPOLLONESHOT case

In commit e8a60448031b07cf7ae9e71ce269ad6eea07c46e (epoll: Do not search
again in EPOLLONESHOT case), when we clear evt.events, we clear a local
copy instead of the one in the map. Fix by taking a reference of
epoll_event.

Passed scripts/run.py -e "/tests/tst-epoll.so" test.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>