Add option ec2.hvm, to set some boot configuration specific for EC2 HVM instances.

(cherry picked from commit 35c76d917307b7ac405486855cfe63021810dba5)

Conflicts:
	nixos/modules/virtualisation/amazon-image.nix
Use HVM instance for EBS creator, guarantees everything is created in the correct zone.

(cherry picked from commit c9763e20e5e793be5eef155601f59dd7639c7ba3)
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.

(cherry picked from commit 044a24e58bcf4cf48df02df936c542839fb08d90)