core: Added QoS overridden member to VdsNetworkInterface

This boolean member represents whether QoS is allowed to be overridden
on the interface, in which case the network won't appear out-of-sync
even if its own QoS configuration is different.

Change-Id: Iaffc4c2f5b35ed34bf64e4d892a0e9ed64f94bde
Signed-off-by: Lior Vernia <lvernia@redhat.com>