error: Print error_report() to stderr if using qmp

monitor_printf will drop the requested output if cur_mon is qmp (for
good reason). However these messages are often helpful for debugging
issues with via libvirt.

If we know the message won't hit the monitor, send it to stderr.

Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

util/qemu-error.c

@@ -20,7 +20,7 @@
  */
 void error_vprintf(const char *fmt, va_list ap)
 {
-    if (cur_mon) {
+    if (cur_mon && !monitor_cur_is_qmp()) {
         monitor_vprintf(cur_mon, fmt, ap);
     } else {
         vfprintf(stderr, fmt, ap);