For in role upgrades to work we need to be able to specify a different
libvirt default file, because newer versions of libvirt have changed the
default file location.
This patch keeps the default as is and the variable should never need to
be changed aside from in the role upgrades (or if you wanted to run
stable/newton with a newer version of libvirt).
Additionally, fix up the libvirt group for the libvirt.conf file.
Change-Id: I4584a8e520e1c6a67c66fbc3ad276b19a78720e3