hw/cxl/mbox: Split mailbox command payload into separate input and output

hw/cxl/mbox: Split mailbox command payload into separate input and output

New CCI types that will be supported shortly do not have a single buffer
used in both directions. As such, split it up. To avoid the complexities
of implementing all commands to handle potential aliasing, take a copy of
the input before use.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20231023160806.13206-3-Jonathan.Cameron@huawei.com>
Reviewed-by: Fan Ni <fan.ni@samsung.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>