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 conductor usage from add_instance_fault_from_exc()"