xenapi: ensure finish_migration cleans on errors

This change makes finish_migration share code with spawn.

This fixes some inconsistencies, such as applying the security
group filters during finish_migration.

Fixes bug 1073303
Fixes bug 1073306

Change-Id: Ib4f2a96618692c141708535f9463c856f3395922
Merge "xenapi: ensure finish_migration cleans on errors"