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
So all the other get methods call the process_collection method so I check for a nil dataset there. However, get_ems does not call proocess_Collection so I had to explicitly check for nil for the ems.