ec2: Add VirtType parameter to RegisterImage

It's now possible to create instance-store HVM AMIs:

    http://sorcery.smugmug.com/2014/01/29/instance-store-hvm-amis-for-amazon-ec2/

This is controlled by a new optional parameter `VirtType` to
`RegisterImage`, which must be either `paravirtual` or `hvm`.