xenapi: move session into new client module

This is the start of building a set of objects on top of the XenAPI
library to make a clear separation between XenAPI client and
business logic.

This is all the first steps towards pull out the client lib into oslo
incubator, so we can share code between Nova and Cinder.

DocImpact
While there is a change to the sample config file, there is no change in
the content of the config file.

Part of blueprint xenapi-driver-refactor
Change-Id: I463923a9ee289ff4b86fcf26d8c656609685857e
Merge "xenapi: move session into new client module"