hw/mem/cxl-type3: Add AER extended capability

hw/mem/cxl-type3: Add AER extended capability

This enables AER error injection to function as expected.
It is intended as a building block in enabling CXL RAS error injection
in the following patches.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Message-Id: <20230302133709.30373-6-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Fan Ni <fan.ni@samsung.com>