Correct test boundary for libvirt_driver.get_info

The test_get_domain_info_with_more_return() unit test was causing
failure on some systems where lxml was not capable of handling None for
the xml to parse. However, our unit tests should not be testing libvirt;
the unit tests in libvirt already do that. Our unit tests should simply
test the code boundaries of the unit of code that they wish to stress,
and not anything further.

This patch changes the unit test in question to only test the code
within the libvirt_driver.LibvirtDriver.get_info() method, and nothing
more, and in doing so, we remove any calls at all to libvirt and mock
out where the code passes the boundaries of the get_info() method.

Change-Id: I022cc331a980c2a4d98f16d2d77b24839b528464
Closes-bug: #1301602
Merge "Correct test boundary for libvirt_driver.get_info"