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.
Make libvirt_cpu_model configurable

Instead of setting the model outright, allow us to change it when
requested only.
Merge pull request #494 from craigtracey/cpu-model

Set default cpu model to Opteron_G3