[product: zaphod][project: zaphod-v0.3]: Implemented checks for running VMs.

SYNOPSIS
[modified]m42/test/test_zaphod.py: Test for running vms.
[modified]m42/zaphod.py: Implemented listing and checking for running VMs.

DETAILS
checking for a running VM is done by calling vboxmanage list running VMs
and extracting a list of running VMs and then checking if the specified VM
name is present in the list. The core code to do this is almost identical
to Zaphod.listvms. So some refactoring was done and the test for
running VMs was implemented analogously to testing for VM existence.
[product: zaphod][project: zaphod-v0.3]: Added docstring for vmexists.

SYNOPSIS
[modified]m42/zaphod.py: Added docstring for Zaphod.vmexists.