Added missing tests for the verify_vbmeta_image methods & fixed bugs.

Test: atest --host aftltool_test
Test: atest --host libavb_host_unittest
Test: ./aftl_integration_test.py
Bug: 147415409
Change-Id: Iecb490ac1598fbf967e82e442c58b4372b2d12ac
diff --git a/avbtool b/avbtool
index 2523449..c20ff0c 100755
--- a/avbtool
+++ b/avbtool
@@ -2577,6 +2577,9 @@
       AvbVBMetaHeader, the third argument is a list of
       AvbDescriptor-derived instances, and the fourth argument is the
       size of |image|.
+
+    Raises:
+      AvbError: In case the image cannot be parsed.
     """
     assert isinstance(image, ImageHandler)
     footer = None
@@ -2797,7 +2800,6 @@
       padding_needed = padded_size - len(vbmeta_blob)
       output.write('\0' * padding_needed)
 
-
   def _generate_vbmeta_blob(self, algorithm_name, key_path,
                             public_key_metadata_path, descriptors,
                             chain_partitions,
@@ -4095,7 +4097,6 @@
     self._add_common_args(sub_parser)
     sub_parser.set_defaults(func=self.make_vbmeta_image)
 
-
     sub_parser = subparsers.add_parser('add_hash_footer',
                                        help='Add hashes and footer to image.')
     sub_parser.add_argument('--image',