Document "options kvm_intel nested=1"
Developers trying to run the test suite from a VM (instead of from their host system, e.g. because they don't run stretch on it, and/or want to separate that away), will likely need to set this option (and reload their KVM module/restart their VMs):
echo "options kvm_intel nested=1" > /etc/modprobe.d/kvm.conf
This should land in some documentation.
Test suite setup doc: better document running in a Stretch VM (refs: #16617).