hw/cxl/mbox: Add support for background operations

hw/cxl/mbox: Add support for background operations

Support background commands in the mailbox, and update
cmd_infostat_bg_op_sts() accordingly. This patch does not implement mbox
interrupts upon completion, so the kernel driver must rely on polling to
know when the operation is done.

Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20231023160806.13206-12-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>