Support VirtIO RNG

[brutesque]

Afaik it is currently not possible to set the VirtIO RNG key in the VM's hardware tab using the terraform module.

There is a command-line equivalent for this.

qm set <vmid> -rng0 source=<source>[,max_bytes=X,period=Y]

It would be nice if it's possible to add a block like this:

resource "proxmox_virtual_environment_vm" "templates" {
  ...
  virtio_rng {
    source    = "/dev/urandom"
    max_bytes = 1024
    period    = 1000
  }
  ...
}