nixos/qemu-vm: fix running VM with QEMU_NET_OPTS

I realized quite recently that running a test VM - as documented in the
manual - like

    QEMU_NET_OPTS='hostfwd=tcp::8080-:80'  ./result/bin/nixos-run-vms

doesn't work anymore on `master`. After bisecting I realized that the
introduction of a forward-port option[1] is the problem since it adds a
trailing comma even if no forwarding options are specified via
`virtualisation.forwardPorts`. In that case, the networking options
would look like `-netdev user,id=user.0,,hostfwd=tcp::8080-:80' which
confused QEMU and thus the VM refused to start.

Now, the trailing comma is only added if additional port forwards are
specified declaratively.

[1] b8bfc81d5b

nixos/modules/virtualisation/qemu-vm.nix

@@ -780,7 +780,7 @@ in
       in
       [
         "-net nic,netdev=user.0,model=virtio"
-        "-netdev user,id=user.0,${forwardingOptions}\${QEMU_NET_OPTS:+,$QEMU_NET_OPTS}"
+        "-netdev user,id=user.0,${forwardingOptions}$QEMU_NET_OPTS"
       ];
 
     # FIXME: Consolidate this one day.