network: change default of forwardPlainNames to 'yes'

The previous patch fixed "forwardPlainNames" so that it really is
doing only what is intended, but left the default to be
"forwardPlainNames='no'". Discussion around the initial version of
that patch led to the decision that the default should instead be
"forwardPlainNames='yes'" (i.e. the original behavior before commit
f3886825). This patch makes that change to the default.

tests/networkxml2xmlupdateout/nat-network-dns-more-hosts.xml

@@ -5,7 +5,7 @@
     <interface dev='eth0'/>
   </forward>
   <bridge name='virbr0' stp='on' delay='0'/>
-  <dns>
+  <dns forwardPlainNames='no'>
     <host ip='f0:d::f0:d'>
       <hostname>pudding</hostname>
     </host>

tests/networkxml2xmlupdateout/nat-network-no-hosts.xml

@@ -5,6 +5,7 @@
     <interface dev='eth0'/>
   </forward>
   <bridge name='virbr0' stp='on' delay='0'/>
+  <dns forwardPlainNames='no'/>
   <ip address='192.168.122.1' netmask='255.255.255.0'>
   </ip>
 </network>