HACKING: Update status of format checking

Hopefully all functions with printf like arguments now use format checking.

This was tested with default build configuration on linux
and windows hosts (including some cross compilations),
so chances are good that there remain few (if any) functions
without format checking.

Therefore the last comment in HACKING is no longer valid but misleading.

Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
