Get images from the source (i.e. blessed) instance

Fixes OS-115

Some of the migration code refactor seemed to change where the
image_refs were retrieved from the blessed instance to the launched
instance. This is incorrect because the image_refs are an attribute
of the blessed instance. This corrects the error.

Change-Id: Ibece5281819fcf76d93dc3ab177a8bc9f29f0566