In order to avoid migration issues, we enable PVH only for
machine type >= 4.0
Suggested-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NStefano Garzarella <sgarzare@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>