random: added a bus interface for RNG modules.

* Yarrow RNG is the default RNG module.
device_manager: tune the search paths

* the Virtio RNG PCI device has the class 0, so can't be found using usual
paths. Add 0 to _AlwaysRegisterDynamic() and "busses/virtio" in _GetNextDriverPath()
for non generic drivers to help finding virtio_pci.
* The RNG Virtio device is generic and needs "busses/random" to find virtio_rng.
random: add a Virtio RNG module

* The default module is replaced by the Virtio RNG module when found.
* This can have the undesired effect of rendering /dev/urandom slow.
* Tested with the following QEmu command line option:
-device virtio-rng-pci,rng=rng0 -object rng-random,filename=/dev/random,id=rng0
* moved random.h to private/drivers headers.
VirtioRNG: add to the image