ec2-release: check image format before upload

EC2 doesn't support image formats other than raw. However it does
not verify image format before upload so qcow2 image may be successfully
imported as a volume and attached to an instance. Instance with such a volume
will fail to start and there will be no indication of what went wrong.

Image format verification added to the ec2-release.sh script to eliminate
such a possibility.

Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>