libqtest: Simplify qmp_fd_vsend() a bit

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: <20180806065344.7103-9-armbru@redhat.com>

tests/libqtest.c

@@ -488,24 +488,20 @@ QDict *qtest_qmp_receive(QTestState *s)
  */
 void qmp_fd_vsend(int fd, const char *fmt, va_list ap)
 {
-    va_list ap_copy;
     QObject *qobj;
 
-    /* qobject_from_jsonv() silently eats leading 0xff as invalid
-     * JSON, but we want to test sending them over the wire to force
-     * resyncs */
+    /*
+     * qobject_from_vjsonf_nofail() chokes on leading 0xff as invalid
+     * JSON, but tests/test-qga.c needs to send that to test QGA
+     * synchronization
+     */
     if (*fmt == '\377') {
         socket_send(fd, fmt, 1);
         fmt++;
     }
 
-    /* Going through qobject ensures we escape strings properly.
-     * This seemingly unnecessary copy is required in case va_list
-     * is an array type.
-     */
-    va_copy(ap_copy, ap);
-    qobj = qobject_from_jsonv(fmt, &ap_copy, &error_abort);
-    va_end(ap_copy);
+    /* Going through qobject ensures we escape strings properly */
+    qobj = qobject_from_vjsonf_nofail(fmt, ap);
 
     /* No need to send anything for an empty QObject.  */
     if (qobj) {