Split xenapi agent code out to nova.virt.xenapi.agent
Split xenapi agent code out to nova.virt.xenapi.agent

This refactoring makes the split between driver methods and internal
methods clearer. It also makes all agent methods private instead of
making some private, some public and some both.

Change-Id: I6c6b405942e47a85816e10224d4e6d986a8b06dc

n0ano/gantt,n0ano/gantt