A lot of virtualization has happened in organizations without any regard for security.  Security professional can continue to warn businesses about some of the impending security issues with virtualization, don't expect them to secure their environments unless and until a system(s) is compromised.
Although all the attacks are possibilities, none of them have actaully happened.  Some of the possibilities are:

As per the following article: http://www.itworldcanada.com/a/Enterprise-Infrastructure/f103508d-d9a0-4f01-bbf3-394577c59ca5.html