qemu will detach usbredir if backend chardev socket disconnect
Description of problem:
When using the usbredir device in the VM, initiate a hot migration to the VM.  
After the migration is completed, the drive letter of the usb in the VM has changed.  
Actually the device has been unplugged and re-plugged in the VM.  
I think we should keep the plugged state of the device after the migration?
Steps to reproduce:
1. Start a usbredirserver `usbredirserver -p 7000 -v 4 5-2`;
2. Start a VM with a usbredir device attached to it;
3. Mount the usb device in the VM;
4. Migrate the VM, after the migration done, wait a minute，the drive letter of the usb in the VM has changed.
Additional information:
I've found this bug https://bugzilla.redhat.com/show_bug.cgi?id=1254971, this is just to allow the chardev to be reconnected in time when it is disconnected.   
Can we make chardev reconnect without unpluging the usbredir device?