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/networkxml2xmltest.c

@@ -127,6 +127,8 @@ mymain(void)
     DO_TEST("empty-allow-ipv6");
     DO_TEST("isolated-network");
     DO_TEST("routed-network");
+    DO_TEST("routed-network-no-dns");
+    DO_TEST_PARSE_ERROR("routed-network-no-dns-extra-elements");
     DO_TEST("open-network");
     DO_TEST_PARSE_ERROR("open-network-with-forward-dev");
     DO_TEST("nat-network");