Add a document with procedure to configure KVM-based nested
virtualization on the physical host and to configure DevStack (in a VM)
to take advantage of it.
Current topics:
- Configure nested virt on Intel hosts
- Configure nested virt on AMD hosts
- Expose virt extensions to DevStack VM
- Ensure DevStack VM is using KVM
Change-Id: Ibe6fa482cc0d51183438d99680a0e10d0da652cb