virtio: console: replace EMFILE with EBUSY for already-open port

Returning EMFILE (process has too many open files) is incorrect to
indicate a port is already open by another process.  Use EBUSY for that.

This does change what we report to userspace, but I believe userspace
can look at it this way: it gets EBUSY, a new error code, instead of
EMFILE.  It's still an error, and that's not changing.
Reported-by: Mateusz Guzik <mguzik@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/char/virtio_console.c

@@ -1036,7 +1036,7 @@ static int port_fops_open(struct inode *inode, struct file *filp)
 	spin_lock_irq(&port->inbuf_lock);
 	if (port->guest_connected) {
 		spin_unlock_irq(&port->inbuf_lock);
-		ret = -EMFILE;
+		ret = -EBUSY;
 		goto out;
 	}