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).

tests/networkxml2confdata/routed-network-no-dns.conf

@@ -0,0 +1,11 @@
+##WARNING:  THIS IS AN AUTO-GENERATED FILE. CHANGES TO IT ARE LIKELY TO BE
+##OVERWRITTEN AND LOST.  Changes to this configuration should be made using:
+##    virsh net-edit local
+## or other application using the libvirt API.
+##
+## dnsmasq conf file created by libvirt
+strict-order
+port=0
+except-interface=lo
+bind-dynamic
+interface=virbr1

tests/networkxml2confdata/routed-network-no-dns.xml

@@ -0,0 +1,10 @@
+<network>
+  <name>local</name>
+  <uuid>81ff0d90-c91e-6742-64da-4a736edb9a9b</uuid>
+  <bridge name="virbr1"/>
+  <mac address='12:34:56:78:9A:BC'/>
+  <forward mode="route" dev="eth1"/>
+  <dns enable='no'/>
+  <ip address="192.168.122.1" netmask="255.255.255.0">
+  </ip>
+</network>