virtio-mmio is still used by default, so if PCI is desired
it's necessary to explicitly opt-in by adding an appropriate

  <address type='pci' domain='0x0000' ... />

element to the corresponding device.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>