[zaphod-v0.3]: Increase stop VM poweroff wait time.

SYNOPSIS
[modified]m42/zaphod.py: Increase stopvm poweroff wait.
[modified]m42/test/test_zaphod.py: Report errors along with execute asserts.
[modified]m42/test/zaphodtestutil.py: Inc poweroff wait, Impl. execnatcmd cmd.

DETAILS
Increase post-poweroff wait time to handle slow shutdowns on older Macs
running OS X. Also implemented a command line option on zaphodtestutil to
execute a command on a NATed VM remotely. This allows interactive
debugging for ssh problems.