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
Merge "xenapi: make session calls more discoverable"