ovs-vtep: Don't flood to multiple 'uknown-dsts'.

ovs-vtep: Don't flood to multiple 'uknown-dsts'.

This commit changes VTEP simulator's flooding logic as follows:

1. Traffic coming from a VTEP physical port will only be flooded
to all the physical ports in the same VTEP that belongs to that logical
switch and to ONLY one of the devices that have been marked as 'unknown-dst'

2. Traffic coming from any device that has 'unknown-dst' marked, will only be
flooded by VTEP to its physical ports that belong to that logical switch.

Bug #20144.
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>