selftests/memory-hotplug: Adjust log info for maintainability

Redirect misleading error message to /dev/null for
offline_memory_expect_success(), And, add an output
for online->offline test.

Signed-off-by: Zhao Gongyi <zhaogongyi@huawei.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>