cache_fetch: Fix MI tree format

The MI "cache_fetch" command was building a poorly formatted MI tree
(viz below). We fix the command tree format in order to make the
resulting data easier to digest programatically.

Old MI tree format visualization:
    'NULL: "out_key: [out_value]"'

New MI tree format visualization:
    '"key": "out_key", "value": "out_value"'

Reported by Hamid Elaosta.
Fixes #1298

mi/mi_core.c

@@ -511,8 +511,10 @@ static struct mi_root *mi_cachefetch(struct mi_root *cmd, void *param)
 		goto done;
 	}
 
-	addf_mi_node_child( &rpl_tree->node, 0, 0, 0, "%.*s = [%.*s]", attr.len,
-			attr.s, value.len, value.s);
+	addf_mi_node_child(&rpl_tree->node, 0, "key", 3, "%.*s",
+	                   attr.len, attr.s);
+	addf_mi_node_child(&rpl_tree->node, 0, "value", 5, "%.*s",
+	                   value.len, value.s);
 
 	pkg_free(value.s);