s390x: Library resources for CSS tests

Provide some definitions and library routines that can be used by
tests targeting the channel subsystem.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Janosch Frank <frankja@de.ibm.com>
Message-Id: <1594904550-32273-1-git-send-email-pmorel@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>