qemu-char: do not forward events through the mux until QEMU has started

Otherwise, the CHR_EVENT_OPENED event is sent twice: first when the
backend (for example "stdio") is opened, and second after processing
the command line.

The incorrect sending of the event prints the monitor banner when
QEMU is started with "-serial mon:stdio".  This includes the "(qemu)"
prompt; thus the monitor seems to be dead, whereas actually the
active front-end is the serial port.
Reported-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Tested-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

qemu-char.c

@@ -735,19 +735,23 @@ static void mux_chr_read(void *opaque, const uint8_t *buf, int size)
         }
 }
 
+static bool muxes_realized;
+
 static void mux_chr_event(void *opaque, int event)
 {
     CharDriverState *chr = opaque;
     MuxDriver *d = chr->opaque;
     int i;
 
+    if (!muxes_realized) {
+        return;
+    }
+
     /* Send the event to all registered listeners */
     for (i = 0; i < d->mux_cnt; i++)
         mux_chr_send_event(d, i, event);
 }
 
-static bool muxes_realized;
-
 /**
  * Called after processing of default and command-line-specified
  * chardevs to deliver CHR_EVENT_OPENED events to any FEs attached