docs: Add missed RNG schema for interface

We support <interface> of type "mcast", "server", and "client",
but the RNG schema for them are missed. Attribute "address" is
optional for "server" type. And these 3 types support
<mac address='MAC'/>, too.

tests/qemuxml2argvdata/qemuxml2argv-net-server.xml

@@ -0,0 +1,29 @@
+<domain type='qemu'>
+  <name>QEMUGuest1</name>
+  <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
+  <memory>219136</memory>
+  <currentMemory>219136</currentMemory>
+  <vcpu>1</vcpu>
+  <os>
+    <type arch='i686' machine='pc'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu</emulator>
+    <disk type='block' device='disk'>
+      <source dev='/dev/HostVG/QEMUGuest1'/>
+      <target dev='hda' bus='ide'/>
+      <address type='drive' controller='0' bus='0' unit='0'/>
+    </disk>
+    <controller type='ide' index='0'/>
+    <interface type='server'>
+      <mac address='52:54:00:8c:b9:05'/>
+      <source address='192.168.0.1' port='5558'/>
+    </interface>
+    <memballoon model='virtio'/>
+  </devices>
+</domain>