virtio-rng: fix incorrect use of valarray

std::valarray does not guarantee its elements will be allocated contiguously,
so the form &v[0] is only guaranteed to point to the first element, not the
rest.

Switch to std::vector, where contiguity is guaranteed.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>