Fix: force the poll() return value to be nb_fd

With poll(), we have to iterate over all fd in the pollset since it is
handled in user space where we don't have to with epoll.o

This is a first patch to fix the fact that we should iterate over the
number of fd the lttng_poll_wait() call returns which is for epoll the
number of returned events and with poll the whole set of fd.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>