Enable fake driver can live migration

Now two methods of virt/fake.py does not implemented
1) ensure_filtering_rules_for_instance
2) unfilter_instance

Also the method live_migration does not call
post_live_migration_at_destination, this will cause compute
driver has no chance to update task state for the VM in DB.

Fix bug 1223090

Change-Id: I2fe552faa5f73b47b97e8fefc5c4de04d6785f8c
Merge "Enable fake driver can live migration"