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
Add create() method to InstanceFault object

This allows creating InstanceFaults through the object interface.

Related to blueprint virt-objects-juno

Change-Id: Iedde2e220127778fa6252d310f9e73c8506f605e
Remove conductor usage from add_instance_fault_from_exc()

This makes add_instance_fault_from_exc() use the InstanceFault
object instead of a raw conductor call. This is actually a
minor change except that all callers of this method must now
stop passing the conductor API, and tests must return something
valid from the mocked call.

Related to blueprint virt-objects-juno

Change-Id: I49b17c1ef3d14fd36f016c37d88287e62126bf68
Merge "Remove use of service_* conductor calls from xenapi host.py"
Merge "Add create() method to InstanceFault object"
Merge "Remove conductor usage from add_instance_fault_from_exc()"