XenServer has released os-xenapi lib on pypi, this patch is to
use os-xenapi in nova project. In this patch, we only change
the usage of XenAPI to os-xenapi and fix unit tests. In the next
patch, we will delete those unused files.
Partially-Implements: blueprint add-os-xenapi-library
Depends-On: Ic327135b893a77672fd42af919f47f181e932773
Change-Id: I424dfcd507c4b7fbeea5245cc1c234dec01d5781