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...

(cherry picked from commit 54da9cc)

nixos/modules/virtualisation/amazon-image.nix

@@ -33,7 +33,7 @@ let cfg = config.ec2; in
         config.boot.kernelPackages.ena
       ];
     boot.initrd.kernelModules = [ "xen-blkfront" "xen-netfront" ];
-    boot.initrd.availableKernelModules = [ "ixgbevf" "ena" ];
+    boot.initrd.availableKernelModules = [ "ixgbevf" "ena" "nvme" ];
     boot.kernelParams = mkIf cfg.hvm [ "console=ttyS0" ];
 
     # Prevent the nouveau kernel module from being loaded, as it