io: add a QIOChannelNull equivalent to /dev/null

This is for code which needs a portable equivalent to a QIOChannelFile
connected to /dev/null.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

include/io/channel-null.h

@@ -0,0 +1,55 @@
+/*
+ * QEMU I/O channels null driver
+ *
+ * Copyright (c) 2022 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#ifndef QIO_CHANNEL_FILE_H
+#define QIO_CHANNEL_FILE_H
+
+#include "io/channel.h"
+#include "qom/object.h"
+
+#define TYPE_QIO_CHANNEL_NULL "qio-channel-null"
+OBJECT_DECLARE_SIMPLE_TYPE(QIOChannelNull, QIO_CHANNEL_NULL)
+
+
+/**
+ * QIOChannelNull:
+ *
+ * The QIOChannelNull object provides a channel implementation
+ * that discards all writes and returns EOF for all reads.
+ */
+
+struct QIOChannelNull {
+    QIOChannel parent;
+    bool closed;
+};
+
+
+/**
+ * qio_channel_null_new:
+ *
+ * Create a new IO channel object that discards all writes
+ * and returns EOF for all reads.
+ *
+ * Returns: the new channel object
+ */
+QIOChannelNull *
+qio_channel_null_new(void);
+
+#endif /* QIO_CHANNEL_NULL_H */