util: epoll_run: allow interrupting event polling

If an event handler returns a positive value, the event polling and
handling loop is interrupted after all the pending events indicated by
epoll_wait are processed.

travis-ci: success for lazy-pages: add non-#PF events handling
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>