Amazon image: Add NVMe support to the initrd

This is required by the new c5.* instance types.

Note that this changes disk names from /dev/xvd* to
/dev/nvme0n*. Amazon Linux has a udev rule that calls a Python script
named "ec2nvme-nsid" to create compatibility symlinks. We could use
that, but it would mean adding Python to the AMI closure...