Record file system block size where known (#760709)

Record the file system block size in the Partition object.  Only
implemented for file systems when set_used_sectors() method has already
parsed the value or can easily parse the value from the existing
executed command(s).

Needed for ext2/3/4 copies and moves performed using e2image so that
they can be tracked in bytes by the ProgressBar class as e2image reports
progress in file system block size units.

Bug 760709 - Add progress bars to XFS and EXT2/3/4 file system specific
             copy methods