vfio-ccw: Destroy kmem cache region on module exit

Free the vfio_ccw_cmd_region on module exit.

Fixes: d5afd5d1

 ("vfio-ccw: add handling for async channel instructions")
Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
Message-Id: <c0f39039d28af39ea2939391bf005e3495d890fd.1559576250.git.alifm@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>