Enable transitional virtio devices by default.
Enable virtio-1.0 for devices plugged into
PCIe ports (Root ports or Downstream ports).

Using the virtio-1 mode will remove the limitation
of the number of devices that can be attached to a machine
by removing the need for the IO BAR.
Signed-off-by: NMarcel Apfelbaum <marcel@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NCornelia Huck <cornelia.huck@de.ibm.com>