chardev: use backend chr context when watch for fe

In commit 6bbb6c06 ("chardev: use per-dev context for
io_add_watch_poll", 2017-09-22) all the chardev watches are converted to
use per-chardev gcontext to support chardev to be run outside default
main thread.  However that's still missing one call from the frontend
code.  Touch that up.
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: NPeter Xu <peterx@redhat.com>
Message-Id: <20180104141835.17987-2-peterx@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

chardev/char-fe.c

@@ -356,7 +356,7 @@ guint qemu_chr_fe_add_watch(CharBackend *be, GIOCondition cond,
     }
 
     g_source_set_callback(src, (GSourceFunc)func, user_data, NULL);
-    tag = g_source_attach(src, NULL);
+    tag = g_source_attach(src, s->gcontext);
     g_source_unref(src);
 
     return tag;