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.

Feature Branch: https://salsa.debian.org/tails-team/tails/merge_requests/37#note_108671

Related issues

• Related to #15864

Original created by @CyrilBrulebois on 16617 (Redmine)