spapr: add machine type specific instance init function This patches adds machine type specific instance initialization functions. Those functions will be used by following patches to compat class properties for legacy machine types. 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>