Address XSA-99 issue: the event ring is still accessible by the guest

This PR addresses an issue brought up in XSA-99 (http://xenbits.xenproject.org/xsa/advisory-99.html). If the event ring is not removed from the guest physmap, a malicious guest can feed invalid data into it or corrupt it otherwise.

Interestingly, the patch supplied with the XSA will move some of the code into libxc as a helper script. It will ultimately make the code required to use events a bit cleaner, but for us if we want to keep supported legacy xen version, it's going to get ugly. It might be time to think about how to _cleanly_ support multiple version of xen events as the many ifdefs make the code somewhat unreadable.