KVM: selftests: Convert s390's memop test to printf style GUEST_ASSERT

Convert s390's memop test to printf-based GUEST_ASSERT, and
opportunistically use GUEST_FAIL() to report invalid sizes.

Link: https://lore.kernel.org/r/20230729003643.1053367-17-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>