Apart from being shorter, this is also another step away from the
overloading of the term 'image' in the image backend. An 'image' in
this code refers either to the thing which glance stores, or an
instance's disk. Image in the latter context has come about because
that's what libvirt calls it. However, as it's overloaded in nova we
should never have used it. At some point I'd like to rename these all
to 'disk'.
This patch also renames 2 variables called 'backend' to 'root_disk',
to more accurately reflect their meaning. Changing the name of
check_image_exists() made these uses harder to read when the meaning
of the variable wasn't clear.
Change-Id: Ic15e8412971b2a3f8e87ec68605c4753e23f94b5
c859509f8c