README: add libvirt-python dependency
    
    Testcloud requires this during dependency installation. But if you try
    to install it from pypi, you also need gcc and libvirt-devel. It seems
    easier to recommend libvirt-python from rpm right in the readme.
    
    Also, bump testcloud in requirements.txt to the one with fixed deps.
    
    Differential Revision: https://phab.qa.fedoraproject.org/D1199