libqtest: handle zero length memwrite/memread

Some recently added tests pass a zero length to qtest_memwrite().
Unfortunately, the qtest protocol doesn't implement an on-the-wire
syntax for zero-length writes and the current code happily sends
garbage to QEMU. This causes intermittent failures.

It isn't worth the pain to enhance the protocol, so this patch
simply fixes the issue by "just return, doing nothing". The same
fix is applied to qtest_memread() since the issue also exists in
the QEMU part of the "memread" command.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 148412457273.22750.983275587432075569.stgit@bahia
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>