Add RNG device to QEMU config

Newer kernels (>=4.18) are far more stringent regarding the generation of random numbers. This results in boot taking much longer when there are limited sources of good entropy.

Tweak the qemu configuration to add a source of entropy so that we aren't starved of random numbers during boot.

Signed-off-by: Martyn Welch martyn.welch@collabora.com