epoll: improve tracepoints

Since we keep the fd, we may as well emit it in tracepoints as it is more
useful than the file pointer.  Also provide the event information.

Reviewed-by: Tomasz Grabiec <tgrabiec@gmail.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
epoll: fix deadlock in socket_file::epoll_add()

The normal lock order is the socket lock, then file::f_lock, but
socket_file::epoll_add() inverts that order.

Fix by moving things around to use the expected order.

Fixes wrk running in osv (as a client).

Reviewed-by: Tomasz Grabiec <tgrabiec@gmail.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>