Let's add a new abstract "virtio memory device" type, and use it as
parent class of virtio-mem-pci and virtio-pmem-pci.

Message-ID: <20230711153445.514112-2-david@redhat.com>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>