hw/pci-bridge/cxl_upstream: Move defintion of device to header.

hw/pci-bridge/cxl_upstream: Move defintion of device to header.

To avoid repetition of switch upstream port specific data in the
CXLDeviceState structure it will be necessary to access the switch USP
specific data from mailbox callbacks. Hence move it to cxl_device.h so it
is no longer an opaque structure.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Fan Ni <fan.ni@samsung.com>
Message-Id: <20231023160806.13206-6-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>