qemu: Simplify vmArguments()

Signed-off-by: Pekka Enberg <penberg@iki.fi>
qemu: Enable virtio-rng for versions 1.3 and higher

Virtio-rng support was introduced in QEMU version 1.3 so enable it only
if we detect such a version or higher. This fixes VM launching on older
QEMU versions.

Fixes #105.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
Merge pull request #104 from penberg/qemu/virtio-rng-1.3

Enable virtio-rng only on QEMU 1.3 and higher