hw/cxl/mbox: Generalize the CCI command processing

hw/cxl/mbox: Generalize the CCI command processing

By moving the parts of the mailbox command handling that are CCI type
specific out to the caller, make the main handling code generic. Rename it
to cxl_process_cci_message() to reflect this new generality.

Change the type3 mailbox handling (reused shortly for the switch
mailbox CCI) to take a snapshot of the mailbox input data rather
than operating on it in place.  This reduces the chance of bugs
due to aliasing going forwars.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20231023160806.13206-5-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>