tests: make virtTests run on RHEL/CentOS

This patch includes fixes to let the virt functional test run
on RHEL/CentOS 6.x:

* add /sbin to mkinitrd search path
* fix usage of string formmatting, and to be python 2.6 friendly
  along the way.

Change-Id: I248acbf03fa86585b74e3c6e8c76afe31561663d
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26198
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>