for those trying to avoid this performance and administration nightmare:
1. consolidate your databases onto like instances for workload type
2. consolidate Instances onto clustered physical servers
3. have plan to implement 2012 geo-clustering for site recovery
by the time you finish step 1, you will no longer be a candidate for vm in all but the smallest environments. Many DBA's have allowed vm to ruin their environments, which they will ulitmately have to undo as the pendulum swings back the other direction, due almost entirely to the proliferation of small instances of SQL Server running on individual small servers.