Remove use of service_* conductor calls from xenapi host.py

This removes use of some service calls in the conductor by xenapi's
host.py module and replaces them with object usage. Existing tests
are sufficient to confirm the same behavior remains.

Related to blueprint virt-objects-juno

Change-Id: I908cca4fa1d297c5c9b1d84ae7ebe7452a30ba12
Merge "Remove use of service_* conductor calls from xenapi host.py"