After upgrade from OpenXT 8 to OpenXT 9, existing HVMs with stubdomain and encrypted VHD will fail to boot with the following message:
The crypto keys are present and labelled correctly.
Using `tap-ctl` to try and mount the disk succeeds:
... but the content of the tapdev looks encrypted still.
Install OpenXT 8 (e.g, build 1909).
Install Windows 1903 HVM guest with disk encryption enabled.
Verify that the guest boot correctly, optionally install OpenXT PV tools.
OTA upgrade to OpenXT 9 (e.g, build 6751).
Start the guest installed in OpenXT 8 from the UI.
The guest should start normally.