Address two problems handling the BAT for Checkpoint disks:

Depending upon the disk an attempt may be made to
access BAT information prior to initializing it, so initialize when appropriate.

Based on the VHDX spec the size of the BAT for checkpoint disks is:

Total BAT Entries = Sector Bitmap Blocks Count * (Chunk Ratio + 1)

but there are checkpoint disks where the Sector Bitmap Blocks count is 0.

This is because the calculation of Sector Bitmap Blocks count (as well as
Data Blocks count) was using Integer division and ".ceil", resulting in an
incorrect value.