Add LIBVIRT_CPU_MODE to set CPU mode

In same cases, the hypervisor presents to the guest OS a named CPU model
is similar to the host CPU and adds extra features to approximate the
host model. However, this does not guarantee all features will be
precisely match.

This patch adds LIBVIRT_CPU_MODE to allow users to define the CPU mode
they want to use, for example "host-passthrough".

Change-Id: I83792c776b50d1d22584be2a37cc6a166f09c72b
diff --git a/stackrc b/stackrc
index 2d3a599..d581e48 100644
--- a/stackrc
+++ b/stackrc
@@ -625,6 +625,7 @@
 case "$VIRT_DRIVER" in
     ironic|libvirt)
         LIBVIRT_TYPE=${LIBVIRT_TYPE:-kvm}
+        LIBVIRT_CPU_MODE=${LIBVIRT_CPU_MODE:-none}
         if [[ "$os_VENDOR" =~ (Debian|Ubuntu) ]]; then
             # The groups change with newer libvirt. Older Ubuntu used
             # 'libvirtd', but now uses libvirt like Debian. Do a quick check