docs: add a serial device with a seclabel example

docs/formatdomain.html.in

@@ -4311,6 +4311,12 @@ qemu-kvm -net nic,model=? /dev/null
       <source path='/dev/pts/3'/>
       <target port='0'/>
     </serial>
+    <serial type='file>
+      <source path='/tmp/file'>
+        <seclabel model='dac' relabel='no'/>
+      </source>
+      <target port='0'>
+    </serial>
     <console type='pty'>
       <source path='/dev/pts/4'/>
       <target port='0'/>