qemu-guest: allow to override security.rngd

... otherwise enabling it causes a merge conflict.

Enabling it was necessary to give enough entropy for the sshd daemon in
my libvirt/nixops VM to generate keys see
https://github.com/NixOS/nixops/issues/1199.

nixos/modules/profiles/qemu-guest.nix

@@ -1,7 +1,7 @@
 # Common configuration for virtual machines running under QEMU (using
 # virtio).
 
-{ ... }:
+{ lib, ... }:
 
 {
   boot.initrd.availableKernelModules = [ "virtio_net" "virtio_pci" "virtio_mmio" "virtio_blk" "virtio_scsi" "9p" "9pnet_virtio" ];
@@ -15,5 +15,5 @@
       hwclock -s
     '';
 
-  security.rngd.enable = false;
+  security.rngd.enable = lib.mkDefault false;
 }