cros_build_lib: Return a list from GetImageDiskPartitionInfo

BUG=None
TEST=Updated unit tests all pass

Change-Id: I25e3f78ebba00f692eb8981f4f7d9fff832feb8e
Reviewed-on: https://chromium-review.googlesource.com/1359008
Commit-Ready: LaMont Jones <lamontjones@chromium.org>
Tested-by: LaMont Jones <lamontjones@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cros_generate_stateful_update_payload.py b/scripts/cros_generate_stateful_update_payload.py
index 9476154..fc034b7 100644
--- a/scripts/cros_generate_stateful_update_payload.py
+++ b/scripts/cros_generate_stateful_update_payload.py
@@ -32,12 +32,10 @@
 
   output_gz = os.path.join(output_directory, _STATEFUL_FILE)
 
-  parts = cros_build_lib.GetImageDiskPartitionInfo(image_path)
-  stateful_num = parts[constants.PART_STATE].number
-
   # Mount the image to pull out the important directories.
   with osutils.TempDir() as stateful_mnt, \
-      osutils.MountImageContext(image_path, stateful_mnt, (stateful_num,)) as _:
+      osutils.MountImageContext(image_path, stateful_mnt,
+                                (constants.PART_STATE,)) as _:
     stateful_dir = os.path.join(stateful_mnt, 'dir-%s' % constants.PART_STATE)
 
     try: