Use target_out_dir rather than root_out_dir for VM images and initrds.

Change-Id: I23c6573d14b871841f8dc5b5a42ec32c381c7fa2
diff --git a/test/hftest/hftest.py b/test/hftest/hftest.py
index 74db312..40aeaca 100755
--- a/test/hftest/hftest.py
+++ b/test/hftest/hftest.py
@@ -84,7 +84,7 @@
     initrd = None
     suite = args.image
     if args.initrd:
-        initrd = os.path.join(args.out, "initrd", args.initrd + ".img")
+        initrd = os.path.join(args.out, "obj", args.initrd, "initrd.img")
         suite += "_" + args.initrd
     log = os.path.join(args.log, suite)
     ensure_dir(log)