ppc: spapr: add 2.4 machine type

The following patches will limit the following things to legacy
machine type:

- maximum number of virtqueues for virtio-pci were limited to 64

Cc: Alexander Graf <agraf@suse.de>
Cc: qemu-ppc@nongnu.org

Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Reviewed-by: Alexander Graf <agraf@suse.de>
