s390x/virtio-ccw: add support for virtio based memory devices

s390x/virtio-ccw: add support for virtio based memory devices

Let's implement support for abstract virtio based memory devices, using
the virtio-pci implementation as an orientation. Wire them up in the
machine hotplug handler, taking care of s390x page size limitations.

As we neither support virtio-mem or virtio-pmem yet, the code is
effectively unused. We'll implement support for virtio-mem based on this
next.

Note that we won't wire up the virtio-pci variant (should currently be
impossible due to lack of support for MSI-X), but we'll add a safety net
to reject plugging them in the pre-plug handler.

Message-ID: <20241219144115.2820241-14-david@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>