Stop depending on sitepackages libvirt-python

We can install libvirt-python bindings from pypi now because the libvirt
devs are supporting pypi packages and the version of libvirt we are
testing nova master against is new enough to link against that pypi
package.

Making this switch will better isolate nova test environments as all
dependencies can be installed without any site packages leaking into
virtualenvs. It will also make the dependency on libvirt-python a bit
more explicit.

Change-Id: I2890702869f05a02ad5e2ecb419db06433231b36
Merge "Stop depending on sitepackages libvirt-python"