Explain cpu_model_extra_flags and nested guest support
Explain cpu_model_extra_flags and nested guest support

In the Configuration Guide's section on KVM:

* expand on the implications of selecting a CPU mode and model
  for live migration,
* explain the cpu_model_extra_flags option,
* discuss how to enable nested guests, and the implications and
  limitations of doing so,
* bump the heading level of "Guest agent support".

Closes-Bug: 1791678
Change-Id: I671acd16c7e5eca01b0bd633caf8e58287d0a913

klmitch/nova,mikalstill/nova,mahak/nova,openstack/nova,mahak/nova,mahak/nova,rahulunair/nova,klmitch/nova,klmitch/nova,mikalstill/nova,mikalstill/nova,rahulunair/nova,openstack/nova,klmitch/nova,openstack/nova,rahulunair/nova