add test for virsh update-device

This patch set is for virsh update-device.
There's one problem to confirm:
When libvirt version is 0.10.2, "virsh update-device" command doesn't support "--persistent" option, intead of "--persistent", it supports "--config".
[root@63beta-kvm cfg]# virsh help update-device
  NAME
    update-device - update device from an XML file

  SYNOPSIS
    update-device <domain> <file> [--config] [--force]

  DESCRIPTION
    Update device from an XML <file>.

  OPTIONS
    [--domain] <string>  domain name, id or uuid
    [--file] <string>  XML file
    --config         affect next boot
    --force          force device update

But when I execute "virsh update-device xmlfile --persistent", it still worked..
Is it a bug or not?