s390x/css: handle emw correctly for tsch

s390x/css: handle emw correctly for tsch

We should not try to store the emw portion of the irb if extended
measurements are not applicable. In particular, we should not surprise
the guest by storing a larger irb if it did not enable extended
measurements.

Cc: qemu-stable@nongnu.org
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
(cherry picked from commit f068d320def7fd83bf0fcdca37b305f1c2ac5413)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>