Reset established vnc connections

Currently a vnc console access token provides unlimited time access to the VM once a connection is established. While the token validity period can be controlled, an established connection cannot be torn without altering either vm state or proxy service.

This has security implications. Suppose a token leaks to an adversary, by means of active snooping or human carelessness, and the adversary makes a connection, she can then passively watch the VM console and gather sufficient information to get an ssh connection. While it is difficult to prevent user errors, Nova can help an alerted user by providing a safety abort mechanism. The aim of this blueprint is to provide such a mechanism to a user to reset all established vnc connections to a given VM.