Hi jmtella,
I just installed a Windows 8.1 Ent Preview x64 guest in WS and ran into the blue screen with the error "DRIVER_POWER_STATE_FAILURE", not "0xc000021a", after installing Tools and reboot. The guest works fine so far after reboot.
I am not sure whether we ran into the same issue. Maybe it is the problem of MS. Anyway, I will report the issue "DRIVER_POWER_STATE_FAILURE". It will be appreciated if you can reproduce your error and share the logs with us.
Thanks.
Regards,
Sophia