hw/machine: Clear out left over CXL related pointer from move of state handling to machines.

This got left behind in the move of the CXL setup code from core
files to the machines that support it.

Link: https://gitlab.com/qemu-project/qemu/-/commit/1ebf9001fb2701e3c00b401334c8f3900a46adaa


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20220701132300.2264-2-Jonathan.Cameron@huawei.com>
Acked-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>