XenAPI: support ceilometer service

This to support configuration on ceilometer services for XenAPI.
1. set hypervisor_inspector as xenapi
2. Confiugre the section of [xenapi] for XenAPI connection

For details, please refer to the config doc:
https://docs.openstack.org/ceilometer/latest/configuration/index.html

Change-Id: I4fc649d927031886c694507b3e8a686646a61ef7
blueprint: xenserver-support