This looks like a test case that would be better served by not using
QMPTestCase, but just printing the qemu-img output and having the
message compared against the reference output.
In fact, there is already 049 for testing some qemu-img create options
and we could just add a line there (or multiple lines to cover other
backing file related error cases, too).
Putting it there would both simplify the test code and keep 298 free for
the other series.
None of the above is really a reason to reject the patch. I guess this
is more of a "are you sure? (y/n)" before I apply it. :-)