[ACS44]cherry-pick: dfb59cd6cc0292a88cb619e53f34cdb713879ffd

[Edison Su]

CLOUDSTACK-6464:
The root cause is that, in 3.0.x, if guest network is "vlan://untagged", then 
kvm agent will use whatever value in "private.network.device", while in 4.x, 
kvm agent will use "guest.network.device". So if both value are not the same in 
the agent.properties, then kvm agent will use incorrect bridge to create vif.
The fix will be, kvm agent code needs to honor traffic type passed down from 
mgt server in startcommand, in case of "vlan://untagged".