serial: register vmsd with DeviceClass

serial: register vmsd with DeviceClass

Migration from old to new code works, however the other way fails for
devices that use serial_init/serial_mm_init with "base", used as
instance_id previously.

(with qdev_set_legacy_instance_id, the alias_id is only used in
savevm.c:find_se(), and thus can only be used to match against
"legacy" instance id values. On new code, instance_id is generated
incrementally from 0 with calculate_new_instance_id(), based on
"qdev-path/vmsd-name")

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: xiaoqiang zhao <zxq_yx_007@163.com>