What I really want is a "magic" shell on a VM - i.e. the ability using introspection calls to launch a process on the VM which gives me stdin/stdout, and is running bash or something - but is just magically there via an out-of-band mechanism.
Not really "out of band", but many VMs allow you to setup a serial console, which is sort of that, albeit with a login, but in reality, could create one without one, still have to go through hypervisor auth to access it in all cases, so perhaps good enough for your case?
You can launch KVM/qemu with screen + text console, and just log in there. You can also configure KVM to have a VNC session on launch, and that ... while graphical, is another eye into the console + login.
(Just mentioning two ways without serial console to handle this, although serial console would be fine.)
