Not sure. AWS sent out alerts in early December than many instances needed a reboot, and would be force rebooted on 1/8 iirc. I'm guessing it was to handle patches that were put in place for these issues.
We were naturally first because we took action internally, but also because we didn't need to reboot guests to ship new kernels to hosts. We update kernels on our hosts frequently, and so nobody would have particularly noticed that their VMs underwent migrations (we do it all the time!).
I do not believe that HVM instances (which would be closer to your KVM infrastructure) needed to be rebooted.