integral-image-export.bbclass: Separate manifest into sha256 sums and variables that map build artifacts to variables.

The original manifest was just the sha256 hashes of files. This
is now the manifest.sha256 file. This commit creates a new file
(manifest.vars) that's basically a cheap and ugly way to
communicate state across builds. The recipe that creates and
exports an image now makes the manifest.vars that identifies which
file is the ROOTFS, the INITRAMFS and the KERNEL IMAGE.

This is useful for consumeing recipes. They can use this data to
create configurations etc.
integral-image-ndvm-pkg: Refactor a few variables.
xen-pv-config: Add new class to create PV Xen configs.

Inherit this class in the integral-image-ndvm-pkg recipe to make a Xen config for it.
integral-dom0.inc: Include the NDVM package in the dom0 image.