[Merge] ~davidkrauser/livecd-rootfs/+git/livecd-rootfs:bootable-buildd into livecd-rootfs:ubuntu/master

[David Krauser]

Switched hook to use linux-image-virtual, and saw a substantial size reduction. Also added dbus to the image, which seems to have fixed @saviq's halting issues.
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https://code.launchpad.net/~davidkrauser/livecd-rootfs/+git/livecd-rootfs/+merge/373916
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~davidkrauser/livecd-rootfs/+git/livecd-rootfs:bootable-buildd into livecd-rootfs:ubuntu/master.