libqtest: Enable compile-time format string checking

qtest_qmp() & friends pass their format string and variable arguments
to qobject_from_vjsonf_nofail().  Unlike qobject_from_jsonv(), they
aren't decorated with GCC_FMT_ATTR().  Fix that to get compile-time
format string checking.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <20180806065344.7103-22-armbru@redhat.com>

tests/libqtest.h

@@ -82,7 +82,8 @@ void qtest_quit(QTestState *s);
  *
  * Sends a QMP message to QEMU and returns the response.
  */
-QDict *qtest_qmp(QTestState *s, const char *fmt, ...);
+QDict *qtest_qmp(QTestState *s, const char *fmt, ...)
+    GCC_FMT_ATTR(2, 3);
 
 /**
  * qtest_qmp_send:
@@ -93,7 +94,8 @@ QDict *qtest_qmp(QTestState *s, const char *fmt, ...);
  *
  * Sends a QMP message to QEMU and leaves the response in the stream.
  */
-void qtest_qmp_send(QTestState *s, const char *fmt, ...);
+void qtest_qmp_send(QTestState *s, const char *fmt, ...)
+    GCC_FMT_ATTR(2, 3);
 
 /**
  * qtest_qmpv:
@@ -105,7 +107,8 @@ void qtest_qmp_send(QTestState *s, const char *fmt, ...);
  *
  * Sends a QMP message to QEMU and returns the response.
  */
-QDict *qtest_qmpv(QTestState *s, const char *fmt, va_list ap);
+QDict *qtest_qmpv(QTestState *s, const char *fmt, va_list ap)
+    GCC_FMT_ATTR(2, 0);
 
 /**
  * qtest_qmp_vsend:
@@ -117,7 +120,8 @@ QDict *qtest_qmpv(QTestState *s, const char *fmt, va_list ap);
  *
  * Sends a QMP message to QEMU and leaves the response in the stream.
  */
-void qtest_qmp_vsend(QTestState *s, const char *fmt, va_list ap);
+void qtest_qmp_vsend(QTestState *s, const char *fmt, va_list ap)
+    GCC_FMT_ATTR(2, 0);
 
 /**
  * qtest_receive:
@@ -574,7 +578,7 @@ static inline void qtest_end(void)
  *
  * Sends a QMP message to QEMU and returns the response.
  */
-QDict *qmp(const char *fmt, ...);
+QDict *qmp(const char *fmt, ...) GCC_FMT_ATTR(1, 2);
 
 /**
  * qmp_send:
@@ -584,7 +588,7 @@ QDict *qmp(const char *fmt, ...);
  *
  * Sends a QMP message to QEMU and leaves the response in the stream.
  */
-void qmp_send(const char *fmt, ...);
+void qmp_send(const char *fmt, ...) GCC_FMT_ATTR(1, 2);
 
 /**
  * qmp_receive:
@@ -943,10 +947,10 @@ static inline int64_t clock_set(int64_t val)
 }
 
 QDict *qmp_fd_receive(int fd);
-void qmp_fd_vsend(int fd, const char *fmt, va_list ap);
-void qmp_fd_send(int fd, const char *fmt, ...);
-QDict *qmp_fdv(int fd, const char *fmt, va_list ap);
-QDict *qmp_fd(int fd, const char *fmt, ...);
+void qmp_fd_vsend(int fd, const char *fmt, va_list ap) GCC_FMT_ATTR(2, 0);
+void qmp_fd_send(int fd, const char *fmt, ...) GCC_FMT_ATTR(2, 3);
+QDict *qmp_fdv(int fd, const char *fmt, va_list ap) GCC_FMT_ATTR(2, 0);
+QDict *qmp_fd(int fd, const char *fmt, ...) GCC_FMT_ATTR(2, 3);
 
 /**
  * qtest_cb_for_every_machine: