resize: Refactor the code for creating target partitions.

resize: Refactor the code for creating target partitions.

The old code mixed the business of planning the layout of the target
partitions with the creation of the target partitions.  The
replacement code separates these into two tasks: firstly we create a
new 'partitions' list with the target layout, secondly this directly
drives the creation of the partitions.

As part of this change I have *removed* the old code that was supposed
to handle extended/logical MBR partitions.  It simply didn't work, and
didn't have any hope of working, and there is a separate bug open to
fix it.