1.Listen the event of node detecting or vanishing to create/remove a vxlan tunnel and create/remove a OVS.
2.Listen the event of ovs detecting or vanishing to apply/remove default flows to ovs.
3.Listen the event of host detecting or vanishing to bind vm and tenent. At the same time apply forwarding flows to ovs.
Node and OVS sepretated by device type are stored in device subsystem while VMs as a host are stored in host subsystem.