scripts for running on several OSes within VM

Adding some scripts for automatically creating VMs and running the
whole build machinery inside them. Currently supported:

The `vm-boot-*` pull an base image template (if not cached yet),
create a VM disk (if not existing yet) and calls up the VM in Qemu.
Once it booted, it can be ssh'ed into (w/o any authentication).
Now the corresponding `vm-buildjob-*` script can be used to call
into the VM and run the tool inside it (will be copied into the VM
from the current work dir)

Supported guests:

* FreeBSD 14.2 (ssh port 2022)
* NetBSD 10.1 (ssh port 2023)
* OpenIndia Hipster (ssh port 2024)

For host, only GNU/Linux support so far.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>