xenapi: make _migrate_disk_resizing_up use @step

Make use of the step decorator into _migrate_disk_resizing_up to make it
more consistent with _migrate_disk_resizing_up.

Also add some extra unit tests to help lock down this process with unit
tests.

Part of blueprint xenapi-migrate-ephemeral-disks
Change-Id: I395b8e27380649c667bb8d891802532ee3ab746b