search for: scratch_disk

A lot of code runs 'qemu-img create' or 'truncate' to create blank disk images. In the past I resisted adding an API to do this, since it essentially duplicates what you can already do using other tools (ie. qemu-img). However this does simplify calling code quite a lot since qemu-img is somewhat error-prone to use (eg: don't try to create a disk called "foo:bar")