Factor out add-disk code

The variety of approaches to 'add_disk' are factored out into
a separate function, and Incremental mode benefits by being
closer to supporting the assembly of containers.

Also remove the adding-to-array-data-structure out of sysfs_add_disk
and into add_disk.

And add some tests for --incremental mode to make sure we don't break it.

Signed-off-by: NeilBrown <neilb@suse.de>