cxl: Machine level control on whether CXL support is enabled

cxl: Machine level control on whether CXL support is enabled

There are going to be some potential overheads to CXL enablement,
for example the host bridge region reserved in memory maps.
Add a machine level control so that CXL is disabled by default.

Signed-off-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220429144110.25167-14-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>