tests/iotests: be a little more forgiving on the size test

tests/iotests: be a little more forgiving on the size test

At least on ZFS this was failing as 512 was less than or equal to 512.
I suspect the reason is additional compression done by ZFS and however
qemu-img gets the actual size.

Loosen the criteria to make sure after is not bigger than before and
also dump the values in the report.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Robert Foley <robert.foley@linaro.org>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200225124710.14152-9-alex.bennee@linaro.org>