The s390(x) architecture doesn't feature a PCI bus. For the purpose of
supporting virtio devices a virtual bus called virtio-s390 is used.
A new address type VIR_DOMAIN_DEVICE_ADDRESS_TYPE_VIRTIO_S390 is used to
distinguish the virtio devices on s390 from PCI-based virtio devices.

V3 Change: updated QEMU_CAPS_VIRTIO_S390 to fit upstream.
Signed-off-by: NViktor Mihajlovski <mihajlov@linux.vnet.ibm.com>