Add virttest.staging.backports module

We'll have to stick with python 2.4 for a little while
yet, so let's bring autotest's backport module to virt-test.

This module contains some functions people have been
using to write their tests. In order to use them, you
can just make

from virttest.backports import all, bin, next

The module takes care of figuring out if there's a
builtin present, or if the backported implementation
has to be used.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
tests: Fix usage of functions available only on py versions > 2.4

The virttest.staging.backports module offers backport
implementations of the functions used, so they work under
python 2.4. Even if said tests won't be executed on older
distros, at least pylint checks will pass.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>