VMware: add in test utility to get correct VM backing

Tests were reading the fake backing's but getting the first fake
VM and not the specific fakes VM. This patch adds a method that
returns the running fake VM record.

This is part of blueprint vmware-hot-plug

Change-Id: Ibdc9834e7fdf52019cea9b543dedadf93cdeb224
Merge "VMware: add in test utility to get correct VM backing"