python/ovs/poller: use select.select instead of select.poll.

python/ovs/poller: use select.select instead of select.poll.

eventlet/gevent doesn't work well with select.poll because select.poll blocks
python interpreter as a whole instead of switching from the current thread
which is about to block to other runnable thread.
So ovsdb python binding can't be used with eventlet/gevent.
Emulate select.poll with select.select because using python means that
performance isn't so important.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Ben Pfaff <blp@nicira.com>