KVM: selftests: Convert the memslot performance test to printf guest asserts

Use the printf-based GUEST_ASSERT_EQ() in the memslot perf test instead of
an half-baked open code version.

Link: https://lore.kernel.org/r/20230729003643.1053367-16-seanjc@google.com


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