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Therefore the last comment in HACKING is no longer valid but misleading.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: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

HACKING

@@ -120,6 +120,3 @@ gcc's printf attribute directive in the prototype.
 This makes it so gcc's -Wformat and -Wformat-security options can do
 their jobs and cross-check format strings with the number and types
 of arguments.
-
-Currently many functions in QEMU are not following this rule but
-patches to add the attribute would be very much appreciated.