Fix a bug in XenAPISession's use of virtapi
Fix a bug in XenAPISession's use of virtapi

When the aggregate_get_by_host() call was moved to virtapi,
the XenAPISession's use of it was changed incorrectly. This
fixes that, and also adds a test that would have caught it.

Change-Id: I13ed36efcb7d285e3a99accb007bb0c263b3179c

n0ano/ganttclient