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
Merge pull request #514 from bronaghs/scvmm_null_results

SCVMM: Gracefully handle empty results from Powershell