rng-builtin: add an RNG backend that uses qemu_guest_getrandom()

Add a new RNG backend using QEMU builtin getrandom function.

It can be created and used with something like:

    ... -object rng-builtin,id=rng0 -device virtio-rng,rng=rng0 ...

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20190820160615.14616-2-lvivier@redhat.com>