Remove all direct use of getenv

Remove all direct use of getenv

Unconditional use of getenv is not secure in setuid env.
While not all libvirt code runs in a setuid env (since
much of it only exists inside libvirtd) this is not always
clear to developers. So make all the code paranoid, even
if it only ever runs inside libvirtd.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
(cherry picked from commit 1e4a02bdfe6307f93763fa2c9681f280c564aee5)