Send updated aggregate to compute on add/rm host

The XenAPI driver relies on getting an up-to-date version of the
aggregate during add_host_to_aggregate and remove_host_from_aggregate
This change ensures that happens.

Fixes bug 1161619
Change-Id: I764e1fc3b9ff20b6eda8f6ec00849f2164de0a49
Merge "Send updated aggregate to compute on add/rm host"