hw/cxl-host: Add an index field to CXLFixedMemoryWindow

hw/cxl-host: Add an index field to CXLFixedMemoryWindow

To enable these to be found in a fixed order, that order needs to be known.
This will later be used to sort a list of these structures so that address
map and ACPI table entries are predictable.

Tested-by: Li Zhijian <lizhijian@fujitsu.com>
Reviewed-by: Li Zhijian <lizhijian@fujitsu.com>
Reviewed-by: Fan Ni <fan.ni@samsung.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Tested-by: Itaru Kitayama <itaru.kitayama@fujitsu.com>
Message-id: 20250703104110.992379-2-Jonathan.Cameron@huawei.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>