Fix a race condition adding a security group rule

setup_port_filters() needs to grab self.devices_to_refilter
before it calls prepare_devices_filter(), else it could skip
processing a device if an RPC arrives while it's processing
new devices.  That device will now be handled the next time
it's called.

Bug introduced in commit 3046c4ae22b1

Change-Id: Ib2f460cc095bbea8f9c767dcb9b4d4b66f1a7811
Closes-Bug: 1393925
Merge "Fix a race condition adding a security group rule"