Use autofree heap allocation instead of variable-length array on the
stack. Here we don't expect the bitmap size to be enormous, and
since we're about to read/write it to disk the overhead of the
allocation should be fine.

The codebase has very few VLAs, and if we can get rid of them all 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).

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[PMM: expanded commit message]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-ID: <20230811175229.808139-1-peter.maydell@linaro.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>