KVM: s390: selftests: Add named stages for memop test

The stages synchronize guest and host execution.
This helps the reader and constraits the execution of the test -- if the
observed staging differs from the expected the test fails.

Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Link: https://lore.kernel.org/r/20220308125841.3271721-4-scgl@linux.ibm.com


Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>