Actually..

If you had all of the instance data stored on a fast SAN, so that it was available to all of the hosts simultaneously (possible.)

And you had an ultra-high speed interconnect (40Gbit Infiniband would do) between hosts, for sharing the memory state when you migrate..

4GB of memory, at 40Gbit/s would be transferred in 0.8s. (assuming perfect throughput, all cows are spherical, etc..)