[HaraldMuehlhoffCC]

read_sectors() capped each iteration at a full block worth of sectors, regardless of where in the block the read started, so reads straddling a block boundary continued past the end of the payload block in the file. Each iteration is now limited to the sectors remaining in the current block.

Adds parametrized regression tests based on fixed.vhdx.gz with a distinct fill pattern per payload block and a modified BAT (present→zero, zero→present, not present, non-contiguous payload blocks), read in one go, straddling a single boundary and spanning several blocks. Without the fix, 8 of the new cases fail.

How to test

tox -e lint,py310,py311
tox -e py310 -- tests/disk/test_vhdx.py

Results on Linux with the Git LFS test data: lint OK (ruff check, ruff format --check, vermin), py310 and py311 72 passed each; the VHDX tests alone: 30 passed.

Fixes #92

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