s390x: Add custom pgm cleanup function

Sometimes we need to do cleanup which we don't necessarily want to add
to interrupt.c, so let's add a way to register a cleanup function.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20200807111555.11169-2-frankja@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>