Use HVM instance for EBS creator, guarantees everything is created in the correct zone.
Better support for HVM instances. Now the NixOS images can
be used on HVM instances without needing nixops. Previously
the grub setup was incorrect, so a plain 'nixos-rebuild switch'
and a reboot would result in a broken system.

Also added growing of the partition of the root disk in the initrd,
so you can run resize2fs after initial boot, without needing an
extra reboot. This is useful especially for nixops'
deployment.ec2.ebsInitialRootDiskSize option.