Merge branch 'buildserver-qemu-kvm' into 'master'

Merge branch 'buildserver-qemu-kvm' into 'master'

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.

@mvdan @CiaranG @krt

See merge request !168