qemu: Implement support for default 'random' backend for virtio-rng

qemu: Implement support for default 'random' backend for virtio-rng

This patch implements support for the virtio-rng-pci device and the
rng-random backend in qemu.

Two capabilities bits are added to track support for those:

QEMU_CAPS_DEVICE_VIRTIO_RNG - for the device support and
QEMU_CAPS_OBJECT_RNG_RANDOM - for the backend support.

qemu is invoked with these additional parameters if the device is
enabled:

-object rng-random,id=rng0,filename=/test/phile (to add the backend)
-device virtio-rng-pci,rng=rng0,bus=pci.0,addr=0x4 (to add the device)