virtio: console: Make read() return -ENODEV on hot-unplug

When a port is hot-unplugged while an app was blocked on a read() call,
the call was unblocked but would not get an error returned.

Return -ENODEV to ensure the app knows the port has gone away.
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/char/virtio_console.c

@@ -582,6 +582,9 @@ static ssize_t port_fops_read(struct file *filp, char __user *ubuf,
 		if (ret < 0)
 			return ret;
 	}
+	/* Port got hot-unplugged. */
+	if (!port->guest_connected)
+		return -ENODEV;
 	/*
 	 * We could've received a disconnection message while we were
 	 * waiting for more data.