fix the way we determine a container arch

prior to this change, the container driver_pid (i.e. libvirt_lxc instance) was used, which always returned x86_64
we now use init_pid (i.e. the systemd or upstart or init instance) which now reflects the container's setting
as a side note we could have used instead someting like 'virsh -c lxc:/// dumpxml container_name | grep arch'