It would appear a working solution is to configure removing access to the network service account, what it actually does is to stop the call to VSS, hence no access denied error is reported. The double \\ is the output from the registry export and is required. Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\VSS\VssAccessControl]"NT Authority\\NetworkService"=dword:00000000
I have seen this with a few separate customers now.  Event ID Error 8194 appears on Hyper-V 2012 hosts with the HIT installed. Has anyone seen this?  Volume Shadow Copy Service error: Unexpected error querying for the IVssWriterCallback interface. hr = 0x80070005, Access is denied.. This is often caused by incorrect security settings in either the writer or requestor process. Operation: Gathering Writer Data Context: Writer Class Id: {e8132975-6f93-4464-a53e-1050253ae220} Writer Name: System Writer Writer Instance ID: {c4cafe6b-9e92-49e6-b95e-35b7cd4c0cac}   The forum below intimates this has been seen before and points to the HIT. [URL]  
VSS event ID 8194 after installing HIT 4.6 