[product: zaphod][project: zaphod-v0.3]: Setup NAT when creating a VM.

SYNOPSIS
[modified]m42/test/test_zaphod.py: Setup NAT forwarding in creatvm.
[modified]m42/zaphod.py: Test that NAT forwarding is correctly set up.

DETAILS
Zaphod.createvm now automatically assigns a non-conflicting forwarding port
when creating a VM that uses NAT mode networking. This ensures that
multiple VMs created by zaphod can be started simultaneously and
accessed via ssh.