Add fprintf_function for function pointers to fprintf-like functions

This kind of function pointers is used very often in qemu.

The new data type uses format checking with GCC_FMT_ATTR
and will be used in later patches.

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