scripts: add alternative script to create EC2 AMIs

This script creates AMIs from snapshots instead of running instances, and
does not require uploading and converting the volume.  As a result it is
much, much faster.  The downside is that it must be run on ec2 itself, and
cannot be run on a local host.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>