util: storage: Improve error message when requesting image above 'start'

util: storage: Improve error message when requesting image above 'start'

When a user would specify a backing chain index that is above the start
point libvirt would report a rather unhelpful error:

invalid argument: could not find backing store 1 in chain for 'sub/link2'

This patch adds an explicit check that the index is below start point in
the backing store and reports the following error if not:

invalid argument: requested backing store index 1 is above 'sub/../qcow2' in chain for 'sub/link2'

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1177062