qemu: format server interface without a listen address

https://bugzilla.redhat.com/show_bug.cgi?id=1130390

The listen address is not mandatory for <interface type='server'>
but when it's not specified, we've been formatting it as:
-netdev socket,listen=(null):5558,id=hostnet0
which failed with:
Device 'socket' could not be initialized

Omit the address completely and only format the port in the listen
attribute.

Also fix the schema to allow specifying a model.

docs/schemas/domaincommon.rng

@@ -2207,6 +2207,16 @@
                 <empty/>
               </element>
             </optional>
+            <optional>
+              <element name="model">
+                <attribute name="type">
+                  <data type="string">
+                    <param name='pattern'>[a-zA-Z0-9\-_]+</param>
+                  </data>
+                </attribute>
+                <empty/>
+              </element>
+            </optional>
           </interleave>
         </group>
         <group>

src/qemu/qemu_command.c

@@ -4576,7 +4576,8 @@ qemuBuildHostNetStr(virDomainNetDefPtr net,
     case VIR_DOMAIN_NET_TYPE_SERVER:
        virBufferAsprintf(&buf, "socket%clisten=%s:%d",
                          type_sep,
-                         net->data.socket.address,
+                         net->data.socket.address ? net->data.socket.address
+                                                  : "",
                          net->data.socket.port);
        type_sep = ',';
        break;

tests/qemuxml2argvdata/qemuxml2argv-net-vhostuser.args

@@ -7,4 +7,6 @@ pc -m 214 -smp 1 -nographic -nodefaults -monitor unix:/tmp/test-monitor,server,n
 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:ee:96:6b,bus=pci.0,addr=0x3 \
 -chardev socket,id=charnet1,path=/tmp/vhost1.sock \
 -netdev type=vhost-user,id=hostnet1,chardev=charnet1 \
--device virtio-net-pci,netdev=hostnet1,id=net1,mac=52:54:00:ee:96:6c,bus=pci.0,addr=0x4
+-device virtio-net-pci,netdev=hostnet1,id=net1,mac=52:54:00:ee:96:6c,bus=pci.0,addr=0x4 \
+-netdev socket,listen=:2015,id=hostnet2 \
+-device rtl8139,netdev=hostnet2,id=net2,mac=52:54:00:95:db:c0,bus=pci.0,addr=0x5

tests/qemuxml2argvdata/qemuxml2argv-net-vhostuser.xml

@@ -33,6 +33,11 @@
       <source type='unix' path='/tmp/vhost1.sock' mode='client'/>
       <model type='virtio'/>
     </interface>
+    <interface type='server'>
+      <mac address='52:54:00:95:db:c0'/>
+      <source port='2015'/>
+      <model type='rtl8139'/>
+    </interface>
     <memballoon model='none'/>
   </devices>
 </domain>