theoretically then you could reboot the host os and the VM would remain running even while the host pc was posting the bios.
if it's running on top of the host os, then the vms are only as stable as the host os which isn't good in windows situation.We have disabled the windows update of the host OS and will seldom login to the host OS. Besides, server hardening has been made so that the server os will only have access to the core service and feature to run Hyper-V. So, it is very rare that the host OS is not stable.