SCVMM: Gracefully handle empty results from Powershell


1) Now log the error messages returned from Powershell.
2) In Scvmm.rb - Now check if the hash (@inventory) is empty
3) Now handle empty datasets within the @inventory hash (e.g. vms)

https://bugzilla.redhat.com/show_bug.cgi?id=1134064
Tailing on a block make the code hard to read.  I think you can write this like an assertion above...

```ruby
def process_collection(collection, key)
  @data[key] ||= []
  return unless collection.nil?
  collection.each do |item|
  ...
```