This patch will allow OpenFlow controllers to identify which interface
belongs to a particular VM by using the Domain UUID.

ovs-vsctl get Interface vnet0 external_ids
{attached-mac="52:54:00:8C:55:2C", iface-id="83ce45d6-3639-096e-ab3c-21f66a05f7fa", iface-status=active, vm-id="142a90a7-0acc-ab92-511c-586f12da8851"}

V2 changes:
Replaced vm-uuid with vm-id. There was a discussion in Open vSwitch
mailinglist that we should stick with the same DB key postfixes for the
sake of consistency (e.g iface-id, vm-id ...).