rng: die when trying to pass through disconnected hwrng

rng: die when trying to pass through disconnected hwrng

If /dev/hwrng exists, but no actual generator is connected (or it is
disabled on the host), QEMU will happily start the VM but crash as soon
as the guest accesses the VirtIO RNG device.

To prevent this unfortunate behaviour, check if a useable hwrng is
connected to the host before allowing the VM to be started.

While at it, clean up config_to_command by moving new and existing rng
source checks to a seperate sub.

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