xen: use virDomainDefPostParse for parsing XM/XL/SEXPR cofings

This change ensures to call driver specific post-parse code to modify
domain definition after parsing hypervisor config the same way we do
after parsing XML.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>

tests/sexpr2xmldata/sexpr2xml-no-source-cdrom.xml

@@ -25,11 +25,13 @@
       <source dev='/dev/sda8'/>
       <backingStore/>
       <target dev='hda' bus='ide'/>
+      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
     </disk>
     <disk type='file' device='cdrom'>
       <backingStore/>
       <target dev='hdc' bus='ide'/>
       <readonly/>
+      <address type='drive' controller='0' bus='1' target='0' unit='0'/>
     </disk>
     <interface type='bridge'>
       <mac address='00:16:3e:0a:7b:39'/>
@@ -45,5 +47,6 @@
     <input type='mouse' bus='ps2'/>
     <input type='keyboard' bus='ps2'/>
     <graphics type='vnc' port='-1' autoport='yes'/>
+    <memballoon model='xen'/>
   </devices>
 </domain>