docs: Fix missing slashes in XML examples

When I pasted some XML from libvirt.org, I've found a small mistake.
Signed-off-by: NLuyao Huang <lhuang@redhat.com>

docs/formatdomain.html.in

@@ -3871,7 +3871,7 @@
       <source>
         <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
       </source>
-      <mac address='52:54:00:6d:90:02'>
+      <mac address='52:54:00:6d:90:02'/>
       <virtualport type='802.1Qbh'>
         <parameters profileid='finance'/>
       </virtualport>
@@ -3898,7 +3898,7 @@
   ...
   <devices>
     <interface type='mcast'>
-      <mac address='52:54:00:6d:90:01'>
+      <mac address='52:54:00:6d:90:01'/>
       <source address='230.0.0.1' port='5558'/>
     </interface>
   </devices>
@@ -4772,11 +4772,11 @@ qemu-kvm -net nic,model=? /dev/null
       <source path='/dev/pts/3'/>
       <target port='0'/>
     </serial>
-    <serial type='file>
+    <serial type='file'>
       <source path='/tmp/file'>
         <seclabel model='dac' relabel='no'/>
       </source>
-      <target port='0'>
+      <target port='0'/>
     </serial>
     <console type='pty'>
       <source path='/dev/pts/4'/>