Commit 120345aa attempted to download and build avocado-virt API
documentation together with the avocado API. While this was reliable
offline, it succeeded only on the very first version builds at
readthedocs.org.

So, let's revert back that hackish form of combining the docs. Further
commits will add API documentation generation to avocado-virt itself,
and link avocado and avocado-virt by means of the intersphinx mapping
feature and readthedocs.org sub-project support.
Signed-off-by: NCleber Rosa <crosa@redhat.com>