builder/amazon: configurable virtualization type

This enables building HVM AMIs.

The option is already supported by goamz, I've just exposed the corresponding configuration.

I wasn't sure how to name the property, maybe `virtualization_type` would be better than `ami_virtualization_type`, but I left it as is for consistency.

See [ec2-register](http://docs.aws.amazon.com/AWSEC2/latest/CommandLineReference/ApiReference-cmd-RegisterImage.html).