poll: fix problem with events

I have noticed a bug while testing poll today. While we are constructing the
pfd array, we correctly emplace the fd of the ith element, but events and
revents always comes from the first. That causes problems if the fds in the set
are waiting for different events.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>