virtio: console: Fix crash when port is unplugged and blocked for write

When a program that has a virtio port opened and blocked for a write
operation, a port hot-unplug event will later led to a crash when
SIGTERM was sent to the program. Fix that.
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/char/virtio_console.c

@@ -529,6 +529,10 @@ static bool will_write_block(struct port *port)
 {
 	bool ret;
 
+	if (!port->guest_connected) {
+		/* Port got hot-unplugged. Let's exit. */
+		return false;
+	}
 	if (!port->host_connected)
 		return true;