xenapi: revert on _migrate_disk_resizing_up error

_migrate_disk_resizing_up now follows similar pattern to
_migrate_disk_resizing_down.

This involves using the @step decorator to report progress and rolling
back if any errors occur while migrating the disks to the destination
host.

Part of blueprint xenapi-migrate-ephemeral-disks
Change-Id: I2557357a4f3502f991a5d187c2ecf7ffeb22286e