network: allow disabling dnsmasq's DNS server

If you define a libvirt virtual network with one or more IP addresses,
it starts up an instance of dnsmasq. It's always been possible to
avoid dnsmasq's dhcp server (simply don't include a <dhcp> element),
but until now it wasn't possible to avoid having the DNS server
listening; even if the network has no <dns> element, it is started
using default settings.

This patch adds a new attribute to <dns>: enable='yes|no'. For
backward compatibility, it defaults to 'yes', but if you don't want a
DNS server created for the network, you can simply add:

   <dns enable='no'/>

to the network configuration, and next time the network is started
there will be no dns server created (if there is dhcp configuration,
dnsmasq will be started with "port=0" which disables the DNS server;
if there is no dhcp configuration, dnsmasq won't be started at all).

docs/formatnetwork.html.in

@@ -885,6 +885,18 @@
         information for the virtual network's DNS
         server <span class="since">Since 0.9.3</span>.
 
+        <p>
+          The dns element can have an optional <code>enable</code>
+          attribute <span class="since">Since 2.2.0</span>.
+          If <code>enable</code> is "no", then no DNS server will be
+          setup by libvirt for this network (and any other
+          configuration in <code>&lt;dns&gt;</code> will be ignored).
+          If <code>enable</code> is "yes" or unspecified (including
+          the complete absence of any <code>&lt;dns&gt;</code>
+          element) then a DNS server will be setup by libvirt to
+          listen on all IP addresses specified in the network's
+          configuration.
+        </p>
         <p>
           The dns element
           can have an optional <code>forwardPlainNames</code>

docs/schemas/network.rng

@@ -247,6 +247,11 @@
              and other features in the <dns> element -->
         <optional>
           <element name="dns">
+            <optional>
+              <attribute name="enable">
+                <ref name="virYesNo"/>
+              </attribute>
+            </optional>
             <optional>
               <attribute name="forwardPlainNames">
                 <ref name="virYesNo"/>