KVM: selftests: Add a test for the KVM_S390_MEM_OP ioctl

Check that we can write and read the guest memory with this s390x
ioctl, and that some error cases are handled correctly.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Link: https://lkml.kernel.org/r/20190829130732.580-1-thuth@redhat.com


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