add missing HKLM root to regkey

the chevkm windows psot module had HKLM
missing from the front of one of it's reg key
paths. This was missed in Rails 3 due to the
error being swallowed unexpectedly. in rails 4
we actually see this cause a stack trace

MSP-12384

Author: David Maloney

modules/post/windows/gather/checkvm.rb

@@ -251,7 +251,7 @@ def xenchk(session)
       end
     end
     if not vm
-      srvvals = registry_enumkeys('HARDWARE\ACPI\FADT')
+      srvvals = registry_enumkeys('HKLM\HARDWARE\ACPI\FADT')
       if srvvals and srvvals.include?("Xen")
         vm = true
       end