tools/virtio: add struct cpumask to cpumask.h Add struct cpumask stub used by virtio_config.h. Created using Cursor CLI. Message-ID: <eacf56399ba220513ebcd610f4a5115dc768db80.1764873799.git.mst@redhat.com> Acked-by: Jason Wang <jasowang@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>