Unable to add cpu topology after removal from xml

 After removing cpu topology from xml and saving, user is unable to add it back. Problem existed in 6.5 (possibly before but did not test in 6.4, worked as expected in 6.2.4.) Persists in 6.5.1rc2

Reproduce-able: yes

Create vm and save

Edit vm xml and remove topology

save

edit vm adding in topology

save

edit vm - topology missing