Make XenAPI use Aggregate object

This makes the XenAPI driver use the Aggregate object for its work,
and avoids the need to call back through virtapi to conductor
directly. It also allows us to convert the two aggregate-related
compute manager methods fully to new-world objects.

Related to blueprint compute-manager-objects
Related to blueprint virt-objects

Closes-bug: #1232179
Change-Id: Ib38ab0e4d6feefebda37888f150752167474b693
Merge "Make XenAPI use Aggregate object"