Use --managed-save flag to virsh undefine

If a virtual machine has got a managed save image, it is not
possible to delete it using a plain 'virsh undefine' command.
While Nova doesn't use 'managed save', the libvirt-guests
init script or systemd service may have created one if the
user rebooted their host. Thus we devstack should pass the
--managed-save flag to virsh to ensure the VM's removal.

Change-Id: Id9b072a6dceeb4e179d2b6a25bbdfb559c299e95
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>