Set default cpu model to Opteron_G3

Out of the gate, libvirt provides cpu's as core2duo. This cpu feature
mapping is not very feature rich. For instance, it does not support
nested KVM. Update nova to default to Opteron_G3 for some better cpu
flag support.
Can we add a comment to change this is deploying to non AMD or pre Opterion 2300 CPUs?