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.

Change-Id: Ie8a39147ece3a3c86f4b146fa0af3641b1079319
Merge "Don't use deprecated nova libvirt conf names"