Introduce a symbol which can be used to prevent ccw modules
being loaded into system emulators without ccw support.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Tested-by: Halil Pasic <pasic@linux.ibm.com>
Message-Id: <20210317095622.2839895-3-kraxel@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>