Don't start the lima demo if executed without any cmdline arguments

The old behaviour was confusing to some users. The demo just popping
up and running only for a fraction of second before terminating may
look like a crash to them.

Also while we are at it, add some text message explaining that this
is not the original unmodified memtester.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>