[question]vhost-user: atuo fix network link broken during migration

[yangke (J)]

We find an issue when host mce trigger openvswitch(dpdk) restart in source host 
during guest migration, 
VM is still link down in frontend after migration, it cause the network in VM 
never be up again.

virtio_net_load_device:
    /* nc.link_down can't be migrated, so infer link_down according
     * to link status bit in n->status */
    link_down = (n->status & VIRTIO_NET_S_LINK_UP) == 0;
    for (i = 0; i < n->max_queues; i++) {
        qemu_get_subqueue(n->nic, i)->link_down = link_down;
    }

guset:                    migrate begin -------> vCPU pause --------> vmsate 
load ------->migrate finish
                                            ^                  ^                
     ^
                                            |                  |                
     |
openvswitch in source host:          begin to restart       restarting          
   started
                                            ^                  ^                
     ^
                                            |                  |                
     |
nc in frontend in source:               link down           link down           
  link down
                                            ^                  ^                
     ^
                                            |                  |                
     |
nc in frontend in destination:          link up             link up             
  link down
                                            ^                  ^                
     ^
                                            |                  |                
     |
guset network:                            broken             broken             
   broken
                                            ^                  ^                
     ^
                                            |                  |                
     |
nc in backend in source:                link down          link down            
  link up
                                            ^                  ^                
     ^
                                            |                  |                
     |
nc in backend in destination:           link up             link up             
  link up

The link_down of frontend was loaded from n->status, n->status is link down in 
source, so the link_down of 
frontend is true. The backend in destination host is link up, but the frontend 
in destination host is 
link down, it cause the network in gust never be up again until an guest cold 
reboot.

Is there a way to auto fix the link status? or just abort the migration in 
virtio net device load?