Documentation: Add info on allowing libvirt TCP connections

Deploying worker nodes via the cluster-api with the libvirt actuator
requires the actuator pod to connect to the host's libvirt daemon from
inside the cluster, which requires libvirt to accept TCP connections
from the cluster subnet.  This adds a section to the libvirt
documentation on configuring the daemon to accept TCP connections.

Documentation/dev/libvirt-howto.md

@@ -40,7 +40,50 @@ polkit.addRule(function(action, subject) {
 EOF
 ```
 
-#### 1.6 Prepare the configuration file
+#### 1.6 Configure libvirt to accept TCP connections
+
+The Kubernetes [cluster-api](https://github.com/kubernetes-sigs/cluster-api)
+components drive deployment of worker machines.  The libvirt cluster-api
+provider will run inside the local cluster, and will need to connect back to
+the libvirt instance on the host machine to deploy workers.
+
+In order for this to work, you'll need to enable TCP connections for libvirt.
+To do this, first modify your `/etc/libvirt/libvirtd.conf` and set the
+following:
+```
+listen_tls = 0
+listen_tcp = 1
+auth_tcp="none"
+tcp_port = "16509"
+```
+
+Note that authentication is not currently supported, but should be soon.
+
+In addition to the config, you'll have to pass an additional command-line
+argument to libvirtd. On Fedora, modify `/etc/sysconfig/libvirtd` and set:
+
+```
+LIBVIRTD_ARGS="--listen"
+```
+
+On Debian based distros, modify `/etc/default/libvirtd` and set:
+
+```
+libvirtd_opts="--listen"
+```
+
+Next, restart libvirt: `systemctl restart libvirtd`
+
+Finally, if you have a firewall, you may have to allow connections from the IP
+range used by your cluster nodes.  If you're using the default subnet of
+`192.168.124.0/24`, something along these lines should work:
+
+```
+iptables -I INPUT -p tcp -s 192.168.124.0/24 -d 192.168.124.1 --dport 16509 \
+  -j ACCEPT -m comment --comment "Allow insecure libvirt clients"
+```
+
+#### 1.7 Prepare the configuration file
 1. `cp examples/tectonic.libvirt.yaml ./`
 1. Edit the configuration file:
     1. Set an email and password in the `admin` section
@@ -51,7 +94,7 @@ EOF
     1. Look at the `podCIDR` and `serviceCIDR` fields in the `networking` section. Make sure they don't conflict with anything important.
     1. Set the `pullSecret` to your JSON pull secret.
 
-#### 1.7 Set up NetworkManager DNS overlay
+#### 1.8 Set up NetworkManager DNS overlay
 This step is optional, but useful for being able to resolve cluster-internal hostnames from your host.
 1. Edit `/etc/NetworkManager/NetworkManager.conf` and set `dns=dnsmasq` in section `[main]`
 2. Tell dnsmasq to use your cluster. The syntax is `server=/<baseDomain>/<firstIP>`. For this example:
@@ -60,14 +103,14 @@ echo server=/tt.testing/192.168.124.1 | sudo tee /etc/NetworkManager/dnsmasq.d/t
 ```
 3. `systemctl restart NetworkManager`
 
-#### 1.8 Install the terraform provider
+#### 1.9 Install the terraform provider
 1. Make sure you have the `virsh` binary installed: `sudo dnf install libvirt-client libvirt-devel`
 2. Install the libvirt terraform provider:
 ```sh
 GOBIN=~/.terraform.d/plugins go get -u github.com/dmacvicar/terraform-provider-libvirt
 ```
 
-#### 1.9 Cache terrafrom plugins (optional, but makes subsequent runs a bit faster)
+#### 1.10 Cache terrafrom plugins (optional, but makes subsequent runs a bit faster)
 ```sh
 cat <<EOF > $HOME/.terraformrc
 plugin_cache_dir = "$HOME/.terraform.d/plugin-cache"