Did you ever encounter a qemu process crash? I haven't, but yes, in that case the VM is down should be started automatically.
For your requirement, that a crash of the OS itself is somehow detected is difficult. If the OS reboots itself, you have nothing to do, but if it gets stuck, that's more problematic. I don't know if other hypervisors do this (via some heartbeat over the agent or something), but you can - in my experience - only fix this with a watchdog timer. In Linux, you can simple add the watchdog device from KVM to your machine and monitor the OS within. That works very well, but for Windows, there are no 64-bit drivers available (at least to my knowledge).