tests/qtest/xlnx-versal-trng-test.c: Drop use of variable length array

tests/qtest/xlnx-versal-trng-test.c: Drop use of variable length array

This test program is the last use of any variable length array in the
codebase.  If we can get rid of all uses of VLAs we can make the
compiler error on new additions.  This is a defensive measure against
security bugs where an on-stack dynamic allocation isn't correctly
size-checked (e.g.  CVE-2021-3527).

In this case the test code didn't even want a variable-sized
array, it was just accidentally using syntax that gave it one.
(The array size for C has to be an actual constant expression,
not just something that happens to be known to be constant...)

Remove the VLA usage.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Message-id: 20240125173211.1786196-2-peter.maydell@linaro.org