core: store dom0 info in qubes.xml

At least to have there info about its backup.

This was already done in commit
dc6fd3c8f3, but later was erroneously
reverted during migration to libvirt.

Fixes qubesos/qubes-issues#958

core-modules/006QubesAdminVm.py

@@ -101,9 +101,6 @@ class QubesAdminVm(QubesNetVm):
     def suspend(self):
         return
 
-    def create_xml_element(self):
-        return None
-
     def verify_files(self):
         return True