Re: but surely
VM migration isn't quite as transparent as it could be, which means that AWS / Rackspace customers still have to plan for potential downtime.
Moreover, you need a sufficient number of patched hosts, otherwise have to move VMs from host 'A' to vulnerable host 'B' before you can patch 'A'. Then you have to move the VMs from 'B' back to 'A'. This results in annoyed customers, because their machines were moved twice, rather than once.
And now you have to do all of this within an announced and (hopefully) short maintenance window, so that your customers can make sure all of their devops guys are on deck for it.
The AWS fix for this was spread out over several days, with several multi-hour maintenance windows. It didn't impact us greatly, but it could have been a lot worse. For some customers, it might well have been.