Fixes boot from volume without image

Fixes Bug #1163566

I've also disabled booting from volume with an image specified
although Nova does support this, I've chosen to still disable it in
Horizon because it doesn't make much sense. Tihomir Trifonov also
noted this.

Change-Id: I17f3edbbb0cabe1cf415484f32016aab7a332ed4
Merge "Fixes boot from volume without image"