We will want to poll not only a bunch of uffd-s, but also the lazy
socket, so here's "an fd and a callback" object to be pushed into
epoll.

travis-ci: success for uffd: A new set of improvements
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com>