qemu: add virtio-rng option to GUI

qemu: add virtio-rng option to GUI

Warn the user when selecting /dev/random or disabling speed limits.

'hardware_counts' is used since technically more than one RNG could be
attached to a QEMU machine. It is limited to 1 however, since the
usefulness of such a setup is more than questionable, considering the
linux kernel only ever uses one hwrng at a time anyway.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>