s390x: css: stsch, enumeration test

First step for testing the channel subsystem is to enumerate the css and
retrieve the css devices.

We currently don't enable multiple subchannel sets and therefore only
look in subchannel set 0

This tests the success of STSCH I/O instruction, we do not test the
reaction of the VM for an instruction with wrong parameters.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <1594904550-32273-2-git-send-email-pmorel@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>