hw/cxl: Add a switch mailbox CCI function

hw/cxl: Add a switch mailbox CCI function

CXL switch CCIs were added in CXL r3.0. They are a PCI function,
identified by class code that provides a CXL mailbox (identical
to that previously defined for CXL type 3 memory devices) over which
various FM-API commands may be used. Whilst the intent of this
feature is enable switch control from a BMC attached to a switch
upstream port, it is also useful to allow emulation of this feature
on the upstream port connected to a host using the CXL devices as
this greatly simplifies testing.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20231023160806.13206-7-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>