s390-vfio-ap: introduces s390 kernel debug feature for vfio_ap device driver

Sets up an s390dbf debug log for the vfio_ap device driver for logging
events occurring during the lifetime of the driver.

Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>