conf: fix formatting of udp chardev attributes

It is possible (although possibly not very useful) to leave out
the service attribute when using <source mode='bind'/>

Fix the formatter bug introduced by commit 4a0da345 and format
the host when its present (checked for non-NULL inside
virBufferEscapeString) instead of basing it on the presence
of the service attribute.

https://bugzilla.redhat.com/show_bug.cgi?id=1455825

src/conf/domain_conf.c

@@ -23307,19 +23307,15 @@ virDomainChrSourceDefFormat(virBufferPtr buf,
     case VIR_DOMAIN_CHR_TYPE_UDP:
         if (def->data.udp.bindService || def->data.udp.bindHost) {
             virBufferAddLit(buf, "<source mode='bind'");
-            if (def->data.udp.bindService)
-                virBufferEscapeString(buf, " host='%s'", def->data.udp.bindHost);
-            if (def->data.udp.bindService)
-                virBufferEscapeString(buf, " service='%s'", def->data.udp.bindService);
+            virBufferEscapeString(buf, " host='%s'", def->data.udp.bindHost);
+            virBufferEscapeString(buf, " service='%s'", def->data.udp.bindService);
             virBufferAddLit(buf, "/>\n");
         }
 
         if (def->data.udp.connectService || def->data.udp.connectHost) {
             virBufferAddLit(buf, "<source mode='connect'");
-            if (def->data.udp.connectService)
-                virBufferEscapeString(buf, " host='%s'", def->data.udp.connectHost);
-            if (def->data.udp.connectService)
-                virBufferEscapeString(buf, " service='%s'", def->data.udp.connectService);
+            virBufferEscapeString(buf, " host='%s'", def->data.udp.connectHost);
+            virBufferEscapeString(buf, " service='%s'", def->data.udp.connectService);
             virBufferAddLit(buf, "/>\n");
         }
         break;

tests/genericxml2xmloutdata/generic-chardev-udp.xml

@@ -29,7 +29,7 @@
       <target type='virtio' name='test3'/>
     </channel>
     <channel type='udp'>
-      <source mode='bind'/>
+      <source mode='bind' host='localhost'/>
       <source mode='connect' service='5678'/>
       <target type='virtio' name='test4'/>
     </channel>