kvm tools: use correct error value for virtio-9p RLERROR

RLERROR expects positive error values, passing negative values causes guest kernel
panic.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

virtio/9p.c

@@ -548,7 +548,7 @@ static void virtio_p9_readdir(struct p9_dev *p9dev,
 	fid = get_fid(p9dev, fid_val);
 
 	if (!fid->is_dir) {
-		errno = -EINVAL;
+		errno = EINVAL;
 		goto err_out;
 	}