partitionedfs: separate out partitions placing

partitionedfs: separate out partitions placing

Create a new method which lays out (or places) partitions to the disks. It will
make it possible to find out disk size before creating a disk. Indeed, this
function places all partitions, so the end of the last partition is the size of
the disk. So the user will have to be able to do this:

1. Add all partitions
2. Layout partitions
3. Get the disk size
4. Create the disk
5. Add the disk and format/mount it

And no more buggy heuristics for disk size are needed.

Change-Id: Ib6edf5e57d3d03f4a84d3f46eefad7d25bc793ec
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>