hw/cxl/mailbox: change CCI cmd set structure to be a member, not a reference

hw/cxl/mailbox: change CCI cmd set structure to be a member, not a reference

This allows devices to have fully customized CCIs, along with complex
devices where wrapper devices can override or add additional CCI
commands without having to replicate full command structures or
pollute a base device with every command that might ever be used.

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: <20240523174651.1089554-2-nifan.cxl@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>