Bad openflow rules removal when VM is deleted

To manage broadcast messages (i.e. ARP), some OpenFlow rules are set with multiple actions (i.e. output:5,output:4). But when one of the port defined as output is removed (i.e. when VM shuts down), the OpenFlow rule is deleted. Then the second output can't receive broadcast messages.

It induces strong issues when the second interface is a DHCP server.

To reproduce (tested with Hydrogen):

• launch simultaneously 2 VMs on the same host
• shut down one of these VMs