cmd/otk-gen-partition-table: fix to be wrapped in a "tree" json

This is similar to osbuild#827 but
this time for the `otk-gen-partition-table` which was forgoten
in this other PR :(

cmd/otk-gen-partition-table/main.go

@@ -16,6 +16,11 @@ import (
 	"github.com/osbuild/images/pkg/osbuild"
 )
 
+// Tree is the wrapper aroudn the "real" input
+type Tree struct {
+	Tree Input `json:"tree"`
+}
+
 // Input represents the user provided inputs that will be used
 // to generate the partition table
 type Input struct {
@@ -239,14 +244,14 @@ func run(r io.Reader, w io.Writer) error {
 	/* #nosec G404 */
 	rng := rand.New(rand.NewSource(rngSeed))
 
-	var genPartInput Input
-	if err := json.NewDecoder(r).Decode(&genPartInput); err != nil {
+	var genPartInputTree Tree
+	if err := json.NewDecoder(r).Decode(&genPartInputTree); err != nil {
 		return err
 	}
 	// XXX: validate inputs, right now an empty "type" is not an error
 	// but it should either be an error or we should set a default
 
-	output, err := genPartitionTable(&genPartInput, rng)
+	output, err := genPartitionTable(&genPartInputTree.Tree, rng)
 	if err != nil {
 		return fmt.Errorf("cannot generate partition table: %w", err)
 	}

cmd/otk-gen-partition-table/main_test.go

@@ -173,31 +173,33 @@ func TestUnmarshalOutput(t *testing.T) {
 
 var partInputsSimple = `
 {
-  "properties": {
-    "create": {
-      "bios_boot_partition": true,
-      "esp_partition": true,
-      "esp_partition_size": "2 GiB"
-    },
-    "type": "gpt",
-    "default_size": "10 GiB"
-  },
-  "partitions": [
-    {
-      "name": "root",
-      "mountpoint": "/",
-      "label": "root",
-      "size": "7 GiB",
-      "type": "ext4"
+  "tree": {
+    "properties": {
+      "create": {
+	"bios_boot_partition": true,
+	"esp_partition": true,
+	"esp_partition_size": "2 GiB"
+      },
+      "type": "gpt",
+      "default_size": "10 GiB"
     },
-    {
-      "name": "home",
-      "mountpoint": "/home",
-      "label": "home",
-      "size": "2 GiB",
-      "type": "ext4"
-    }
-  ]
+    "partitions": [
+      {
+	"name": "root",
+	"mountpoint": "/",
+	"label": "root",
+	"size": "7 GiB",
+	"type": "ext4"
+      },
+      {
+	"name": "home",
+	"mountpoint": "/home",
+	"label": "home",
+	"size": "2 GiB",
+	"type": "ext4"
+      }
+    ]
+  }
 }`
 
 // XXX: anything under "internal" we don't actually need to test