I ran across this issue today and thought I would share as I didn't find any immediate results when looking for a solution. I had a VM that was in an unhealthy state where RDP would not connect and the console tab in XOA was blank.
I was in the process of applying updates to the pool and also could not migrate this VM to a different host. I proceeded to install the patches and this VM went offline during the updates after the toolstack restarted.
I tried to boot the VM on the host I had wanted to move it to and got the "SR_BACKEND_FAILURE_46 The VDI is not available not detached cleanly" error. I then tried to just boot the VM with the same result.
The fix was to boot the VM on the original host it had been running on before the shutdown. Hope this helps.