Fix broken API os-migrations

The return is not what we are expecting:
  migration-list: 'unicode' object has no attribute 'iteritems'

The conversion of the object nova.objects.MigrationList into a list
is not applied automatically.

Change-Id: I1579b1baf62275bf448f23be9465d4e9bc11ed85
Closes-Bug: #1260249
Merge "Fix broken API os-migrations"