Add note about vnethost for KVM

The vnethost module can be loaded for added performance, so
created a stub section on KVM performance.

There were some build failures to do with dashboard files
moving around, so fixed those up too.

fixes bug 1064757

Cherry-picked from https://review.openstack.org/32356

Change-Id: Ibcd1797c8db0d0a147a64e567a95ce13f4bd1d3a