qapi/error: Check format string argument in error_*prepend()

error_propagate_prepend() "behaves like error_prepend()", and
error_prepend() uses "formatting @fmt, ... like printf()".
error_prepend() checks its format string argument, but
error_propagate_prepend() does not. Fix by addint the format
attribute to error_propagate_prepend() and error_vprepend().

This would have caught the bug fixed in the previous commit.

Missed in commit 4b57664 "error: Fix use of error_prepend() with
&error_fatal, &error_abort".

Inspired-by: Stefan Weil <[email protected]>
Suggested-by: Eric Blake <[email protected]>
Reviewed-by: Stefan Weil <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <[email protected]>

Author: philmd

include/qapi/error.h

@@ -382,13 +382,15 @@ void error_propagate(Error **dst_errp, Error *local_err);
  * Please use ERRP_GUARD() and error_prepend() instead when possible.
  */
 void error_propagate_prepend(Error **dst_errp, Error *local_err,
-                             const char *fmt, ...);
+                             const char *fmt, ...)
+    GCC_FMT_ATTR(3, 4);
 
 /*
  * Prepend some text to @errp's human-readable error message.
  * The text is made by formatting @fmt, @ap like vprintf().
  */
-void error_vprepend(Error *const *errp, const char *fmt, va_list ap);
+void error_vprepend(Error *const *errp, const char *fmt, va_list ap)
+    GCC_FMT_ATTR(2, 0);
 
 /*
  * Prepend some text to @errp's human-readable error message.