Replace remaining gcc format attributes by macro GCC_FMT_ATTR (format checking)

Replace the remaining format attribute printf by macro
GCC_FMT_ATTR which uses gnu_printf (if supported).

v2
* Removal of dyngen specific code is now done in a separate patch.
* Handle attribute in new ui/spice-display.c, too.

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