Some versions of Windows on some processors access hardware registers that aren’t actually defined for that processor. When they do this, KVM tells them that the instruction isn’t supported, which causes Windows to BSOD. The ignore_msrs option tells it not to do that, but to just ignore it instead.
You can tell whether you need to do this by checking your syslog, which will have lines like “unhandled rdmsr” or “unhandled wrmsr” if your version of Windows is doing it.