Sorry to necro-post this but I thought I'd come back and let you know that the new (finally signed) guest drivers for Windows resolved the problems I had been having with "inaccessible boot device" whenever I tried to install the Xen guest drivers while also using PCI-Passthrough to pass a nvme drive directly to the VM. The VM now boots perfectly fine with the boot disk being a virtual drive while allowing me to use the passthrough nvme as a data disk to which most of the software and my Steam library is installed. And ever since my virtual gaming rig has been stable and performing as I have expected.