buildserver running in qemu/kvm to support KVM on KVM
jenkins.debian.net runs in QEMU/KVM instances, so in order to run the F-Droid buildserver there, it needs to work inside of a KVM guest. The best way I found to do that is to create QEMU/KVM instances via KVM's "nested" virtualization support. This collection of commits enables using QEMU/KVM as the buildserver when ./makebuildserver detects that it is running inside of KVM. Otherwise, the old behavior is default: running in VirtualBox.
I have run these tests inside of ubuntu/16.04 on bare metal, which uses VirtualBox, and ubuntu/16.04 KVM guest, which uses QEMU/KVM. It'll also run on the Guardian Project jenkins box, which is Debian/jessie.