Filter out free regions too small for alignment of partitions.

Make sure that we don't even look at regions shorter than the alignment
grain size, and that free regions have sufficient length after aligning
the start and end sector before instantiating a DiskChunk.

Related: rhbz#1083687
Resolves: rhbz#1147068
Use Decimal for math in Size.convertTo.

Size.__div__ converts the Decimal result to a Size, which rounds its
numerical argument (byte count) down to an integer. Size.convertTo is
meant to retain precision.