docs/cxl: Add an arm/virt example.

docs/cxl: Add an arm/virt example.

Only add one very simple example as all the i386/pc examples will work
for arm/virt with a change to appropriate executable and appropriate
standard launch line for arm/virt. Note that max cpu is used to
ensure we have plenty of physical address space.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
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>
Tested-by: Li Zhijian <lizhijian@fujitsu.com>
Message-id: 20250703104110.992379-5-Jonathan.Cameron@huawei.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>