Object-ify xenapi driver's use of agent_build_get_by_triple()

This makes the xenapi driver use the new Agent object instead of
making a VirtAPI/conductor raw call back to the database.

Related to blueprint virt-objects-juno

Change-Id: I8498c6f3e2fa5d614ac301402d2be778e5edb74e
Merge "Object-ify xenapi driver's use of agent_build_get_by_triple()"