OVS bridge support

[thediveo]

• use bridge NIF type network.BridgeAttrs also for kind "openvswitch"
• in NifAttrs.ResolveRelations, use *network.BridgeAttrs type assertion instead of kind name string matching.
• in UI, use a OVS-specific bridge icon, if possible (though OVS logo doesn't lend itself to the idea of bridge)

nothing beyond this point, no flow displays or the like, just the topology.

[thediveo]

First experiment shows things are not as they seem in OVS land: the master relationship is misappropriated by OVS for something else and thus doesn't give us the topology. OVS documentation is terrible. Another experiment shows that RTNETLINK cannot be used to create OVS bridges and cannot be used to add netdevs as ports. Looks very much like an half-baked kernel integration, at least from the standpoint of an ordinary RTNETLINK user.