Don't add extra disks to booted-from-volume instances

Instances that are booted from volumes (or volume snapshots) have no
local files already, so there's no need to add a root disk for the
image_ref which is not used in the boot-from-volume case.

This makes boot-from-volume work as usual when the always_use_volumes
option is enabled.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>