[libvirt] [PATCH] Fix macvtap device tear down problem on virsh destroy

This patch fixes the problem with the tear down of the macvtap device when issuing a 'virsh destroy' by moving the tear down block past the point of killing the Qemu process. It seems necessary to loop at least once in the case of 'virsh destory' since the device seems to be busy for a while after the Qemu process has been killed. This also still properly tears down the macvtap device when the VM is 'virsh shutdown'ed or halted from inside. Signed-off-by: Stefan Berger <stefanb@us.ibm.com>