I was having this exact same issue but on a brand new VM with a fresh HASSOS qcow2 file (straight from Home Assistant's website) and couldn't for the life of me figure out why it wouldn't boot (since past UEFI boot issues have been due to Home Assistant OS updates breaking its own boot files but this was a fresh image file). I checked the XML view and sure enough it was set to raw instead of qcow2 for some reason. I changed that and it boots up just fine. I wonder what is causing unRAID to pick the wrong disk type.