Updated openstack/openstack
Project: openstack/nova  e3e049f8a28b9d5ee9c112ef7e7aaec7706401f5
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xenapi: make session calls more discoverable

Its currently hard to discover what existing calls there are to
XenAPI, and the *_utils.py are getting very large.

Also, with the move to mock, it is becoming hard work testing multiple
calls to call_xenapi within a single method. Moving to this more object
oriented way of calling XenAPI should make writing tests with mock much
easier, and give us an obvious place to move some of simpler helper
methods in vm_utils and volume_utils.

This is the very first step in this direction. Dependent patches will
show how this can be used.

Part of blueprint xenapi-driver-refactor
Change-Id: I021fbedf2e84d58f24a3d9cb8fab0b906125f7c1