Add format attributes to functions with format strings

Compiling with -Wsuggest-attribute=format shows that these functions
could use a format attribute, which helps the compiler validate format
strings.  This patch adds them.

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Change-Id: I2884d13b08d9ea67680289e59fb40ebdfecdc3d4