Switch from atexit.register() to a more elegant idiom of declaring
resources in a with statement:

  with FilePath('monitor.sock') as monitor_path,
       VM() as vm:
      ...

The files and VMs will be automatically cleaned up whether the test
passes or fails.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20170824072202.26818-4-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>