virttest.libvirt_xml.accessors: Introduce py 2.4 compliant bin()

The builtin bin() was introduced in python 2.6, causing
a problem when running code under python 2.4. So introduce
a bin() implementation equal to the one autotest introduced
recently. However, as autotest 0.15 is still not packaged
by distros, let's use our own copy for now.

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