[PATCH-for-5.1? v2] qapi/error: Check format string argument in error_*prepend()

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

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:

    CC      hw/sd/milkymist-memcard.o
  hw/sd/milkymist-memcard.c: In function ‘milkymist_memcard_realize’:
  hw/sd/milkymist-memcard.c:284:70: error: format ‘%s’ expects a matching ‘char *’ argument [-Werror=format=]
    284 |         error_propagate_prepend(errp, err, "failed to init SD card: %s");
        |                                                                     ~^
        |                                                                      |
        |                                                                      char *

Missed in commit 4b5766488f "error: Fix use of error_prepend() with
&error_fatal, &error_abort".

Inspired-by: Stefan Weil <sw@weilnetz.de>
Suggested-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
Since v1:
- Reword (Markus)
- Add error_vprepend (Stefan)
- Use local style to add the attribute *after* the declaration,
  which is invalid on definition where we get (depending on the
  toolchain):

  . error: attributes should be specified before the declarator in a function definition

  . error: attributes are not allowed on a function-definition

Supersedes: <20200723091309.18690-1-philmd@redhat.com>
---
 include/qapi/error.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/qapi/error.h b/include/qapi/error.h
index 7932594dce..eaa05c4837 100644
--- a/include/qapi/error.h
+++ b/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.
-- 
2.21.3