first cut at optimizing backing creation on migration dest
creating a symlink for all VBR items
added link-to-... soft links into instance directory, to assist with migration
fixed a problem with link-to-... name on destination; eschewing them for cache
migration state only applies to 'running' instances
merged in team/evac
Implements 'rollback' action on migration dest

Fixes EUCA-5443
Merge branch 'team/evac' of git.eucalyptus-systems.com:eucalyptus into team/evac
Merge branch 'dev/dmitrii/sprint5' into dev/dmitrii/evac