This removes the last of the CXL code from the MachineState where it
is visible to all Machines to only those that support CXL (currently i386/pc)
As i386/pc always support CXL now, stop allocating the state independently.

Note the pxb register hookup code runs even if cxl=off in order to detect
pxb_cxl host bridges and fail to start if any are present as they won't
have the control registers available.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Message-Id: <20220608145440.26106-8-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>