amazon-image.nix: Add the ena driver

This is necessary for Enhanced Networking on x1.* instances.

nixos/modules/virtualisation/amazon-image.nix

@@ -20,8 +20,12 @@ let cfg = config.ec2; in
       autoResize = true;
     };
 
-    boot.extraModulePackages = [ config.boot.kernelPackages.ixgbevf ];
-    boot.initrd.kernelModules = [ "xen-blkfront" "xen-netfront" "ixgbevf" ];
+    boot.extraModulePackages =
+      [ config.boot.kernelPackages.ixgbevf
+        config.boot.kernelPackages.ena
+      ];
+    boot.initrd.kernelModules = [ "xen-blkfront" "xen-netfront" ];
+    boot.initrd.availableKernelModules = [ "ixgbevf" "ena" ];
     boot.kernelParams = mkIf cfg.hvm [ "console=ttyS0" ];
 
     # Prevent the nouveau kernel module from being loaded, as it