hw/cxl/mailbox: interface to add CCI commands to an existing CCI

hw/cxl/mailbox: interface to add CCI commands to an existing CCI

This enables wrapper devices to customize the base device's CCI
(for example, with custom commands outside the specification)
without the need to change the base device.

The also enabled the base device to dispatch those commands without
requiring additional driver support.

Heavily edited by Jonathan Cameron to increase code reuse

Signed-off-by: Gregory Price <gregory.price@memverge.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Fan Ni <fan.ni@samsung.com>
Message-Id: <20240523174651.1089554-3-nifan.cxl@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>