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
I might lean towards `warn` here instead of `error`.  I think we should probably reserve `error` for cases where we actually catch an exception.

@Fryguy, thoughts?