This series depends on my virtio-rng series from earlier but goes quite a bit further.
It creates a default backend for the virtio-rng-pci device that will be available if the user doesn't explicitly set one. It also creates a mechanism to provide a simple way to specify default devices via QEMUMachine. Finally, it adds virtio-rng and virtio-balloon to the default machine. The principle here is to try and make a naive execution of qemu as useful as possible. When it comes to virtio-rng, I think it's a lot to ask of a user to try and decide whether they need or not--so let's always make it available. For virtio-balloon, there's really no harm in having it present and it enables ballooning which is a feature most users expect and are aware of. I would expect us to use a similar mechanism to expose a virtio-serial channel for qemu-ga once we plumb guest agent actions through QMP.
