Make xenapi agent code use Instance object for updates

This makes the agent code use the Instance object that it is passed
to do the updates, instead of calling back to the old-style virtapi
instance_update() method.

Related to blueprint virt-objects-juno

Change-Id: Ied8891c974970a25f135a88303789d0c074741ee
Merge "Make xenapi agent code use Instance object for updates"