KVM: s390: selftests: Add macro as abstraction for MEM_OP

In order to achieve good test coverage we need to be able to invoke the
MEM_OP ioctl with all possible parametrizations.
However, for a given test, we want to be concise and not specify a long
list of default values for parameters not relevant for the test, so the
readers attention is not needlessly diverted.
Add a macro that enables this and convert the existing test to use it.
The macro emulates named arguments and hides some of the ioctl's
redundancy, e.g. sets the key flag if an access key is specified.
Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Link: https://lore.kernel.org/r/20220308125841.3271721-3-scgl@linux.ibm.comSigned-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>