apb: convert init to realize

Convert a device model where initialization obviously can't fail,
make it implement realize() rather than init().

Signed-off-by: Wei Jiangang <weijg.fnst@cn.fujitsu.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>