releasetools: Allow verifying chained vbmeta partitions.

Chained vbmeta partitions (vbmeta_system, vbmeta_vendor) were added to
support dynamic partitions. validate_target_files.py misses the logic in
handling such partitions.

Bug: 132882632
Test: Run validate_target_files.py on a target_files.zip that uses
      chained vbmeta_system partition.
Change-Id: Id06c575d13d5e9cc1b621f485ceb75d3e354c39f