qemu-char: ignore flow control if a PTY's slave is not connected

After commit f702e62 (serial: change retry logic to avoid concurrency,
2014-07-11), guest boot hangs if the backend is an unconnected PTY.

The reason is that PTYs do not support G_IO_HUP, and serial_xmit is
never called.  To fix this, simply invoke serial_xmit immediately
(via g_idle_source_new) when this happens.

Tested-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

qemu-char.c

@@ -1168,6 +1168,9 @@ static int pty_chr_write(CharDriverState *chr, const uint8_t *buf, int len)
 static GSource *pty_chr_add_watch(CharDriverState *chr, GIOCondition cond)
 {
     PtyCharDriver *s = chr->opaque;
+    if (!s->connected) {
+        return NULL;
+    }
     return g_io_create_watch(s->fd, cond);
 }
 
@@ -3664,6 +3667,10 @@ int qemu_chr_fe_add_watch(CharDriverState *s, GIOCondition cond,
     }
 
     src = s->chr_add_watch(s, cond);
+    if (!src) {
+        return -EINVAL;
+    }
+
     g_source_set_callback(src, (GSourceFunc)func, user_data, NULL);
     tag = g_source_attach(src, NULL);
     g_source_unref(src);