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
Should this type of check be done for all the `get_` methods?  Or, is `@inventory[:ems]` unique?