OvmfPkg: add driver for Virtio-RNG device

OvmfPkg: add driver for Virtio-RNG device

This adds the new Virtio-RNG DXE module to all three builds of
OvmfPkg. Note that QEMU needs to be invoked with the 'device
virtio-rng-pci' option in order for this device to be exposed to
the guest.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>