Currently nested virtualisation under KVM does not seem to be working in
CI. This breaks the 'bare metal' deployment testing using Tenks, which
lead us to disable it in 749ef824.

This commit forces Tenks to use QEMU for its VMs, allowing us to revert
commit 749ef824..

Change-Id: Id382c218f3b37979341f0d96718a6011a1d9da37
Story: 2005316
Task: 30223