[Merge] livecd-rootfs:default-compat into livecd-rootfs:ubuntu/master

[Brian Murray]

> +1 modulo John Chittum's comments.
> 
> Just one comment/question: should we pin compat=1.1 to we keep building images
> compatible with all the supported Ubuntu releases even if qemu upstream
> changes the default image format?

I'd rather somebody somewhere run into an error with a new compatibility format rather than us having a setting hidden in livecd-rootfs that nobody knows about.
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https://code.launchpad.net/~ubuntu-core-dev/livecd-rootfs/+git/livecd-rootfs/+merge/427296
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