epoll: switch to file based implementation using do_poll()

Instead of using file descriptors and poll(), use do_poll().  This allows
us to get rid of user supplied fds early, which is important as fd lifetime
is decoupled from epoll lifetime.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>