vfio-ccw: Wire in the request callback

The device is being unplugged, so pass the request to userspace to
ask for a graceful cleanup. This should free up the thread that
would otherwise loop waiting for the device to be fully released.

Signed-off-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>