bug 899767: fix vif-plugging with live migration
bug 899767: fix vif-plugging with live migration

Expose vif_unplug method in virt layer, as the compute manager has to
explicitly call it on during live migration, so that network devices are
torn down even if the disk image is not.

Change-Id: Iae88c72f4fd2c7c097c23ef9e5f8fd392052f73e

n0ano/ganttclient