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

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 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: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200723171205.14949-1-philmd@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>