Modify the default cpu-mode for aarh64 in Libvirt

For AArch64, KVM don't recognize the cpu-mode "none",
so change the default cpu-mode as host-passthrough for
generating nova.conf

Change-Id: I94a22e5a15a974b9c11e9f9fd996857453b6e2ca
Signed-off-by: Kevin Zhao <kevin.zhao@linaro.org>
diff --git a/lib/nova_plugins/hypervisor-libvirt b/lib/nova_plugins/hypervisor-libvirt
index d0e364e..51d807a 100644
--- a/lib/nova_plugins/hypervisor-libvirt
+++ b/lib/nova_plugins/hypervisor-libvirt
@@ -55,6 +55,7 @@
     if is_arch "aarch64"; then
         # arm64 architecture currently does not support graphical consoles.
         iniset $NOVA_CONF vnc enabled "false"
+        iniset $NOVA_CONF libvirt cpu_mode "host-passthrough"
     fi
 
     # File injection is being disabled by default in the near future -