Don't use deprecated nova libvirt conf names

libvirt_cpu_mode was renamed to libvirt.cpu_mode
libvirt_type was renamed to libvirt.virt_type

This patch is needed before I346ee0248c03e60716bd7dc4a5ee4493610867cb can
land.

Backported to stable/icehouse so that grenade will work, since we use
nova.conf set by old (stable/icehouse) when upgrading to new. Also this
config was deprecated in icehouse.

Change-Id: Ie8a39147ece3a3c86f4b146fa0af3641b1079319
(cherry picked from commit 1cd8efc89073a2867c8e2bd3a79d6887a180a590)