docs: route element must specify network address

because network address is required by route, so
here we should add one avoid user misunderstand.
Signed-off-by: NChen Fan <chen.fan.fnst@cn.fujitsu.com>

docs/formatdomain.html.in

@@ -4471,7 +4471,7 @@ qemu-kvm -net nic,model=? /dev/null
       <target dev='vnet0'/>
       <b>&lt;ip address='192.168.122.5' prefix='24'/&gt;</b>
       <b>&lt;route family='ipv4' address='192.168.122.0' prefix='24' gateway='192.168.122.1'/&gt;</b>
-      <b>&lt;route family='ipv4' gateway='192.168.122.1'/&gt;</b>
+      <b>&lt;route family='ipv4' address='192.168.122.8' gateway='192.168.122.1'/&gt;</b>
     &lt;/interface&gt;
     ...
     &lt;hostdev mode='capabilities' type='net'&gt;
@@ -4480,7 +4480,7 @@ qemu-kvm -net nic,model=? /dev/null
       &lt;/source&gt;
       <b>&lt;ip address='192.168.122.6' prefix='24'/&gt;</b>
       <b>&lt;route family='ipv4' address='192.168.122.0' prefix='24' gateway='192.168.122.1'/&gt;</b>
-      <b>&lt;route family='ipv4' gateway='192.168.122.1'/&gt;</b>
+      <b>&lt;route family='ipv4' address='192.168.122.8' gateway='192.168.122.1'/&gt;</b>
     &lt;/hostdev&gt;
 
   &lt;/devices&gt;