This reduces the VM downtime

CP-11841: 
vm-migrate downtime: plug the VIFs of the new domain before suspending the old domain
https://github.com/xapi-project/xenopsd/pull/205

This reduces the VM downtime (when both old and new domains are paused) in 0.3s
for each VIF connected to the VM.

ダウンタイムの削減。