Add cpu_mode in quickstart guide

With libvirt 2.0, when using qemu cpu mode is not properly
evaluated and need to be set to cpu_mode = none.

Add this option when kvm is not supported, otherwise user
will still facing errors when launching instances over qemu.

Change-Id: Ic970e3de74817ce4e17bb4e15d130e1e82d595cc

doc/quickstart.rst

@@ -609,6 +609,7 @@ the content shown below.
     cat << EOF > /etc/kolla/config/nova/nova-compute.conf
     [libvirt]
     virt_type = qemu
+    cpu_mode = none
     EOF
 
 For *all-in-one* deployments, the following commands can be run. These will