multi backends: factorize code between single and multi backends

Code is duplicated between single volume backend
(cinder::volume::*) and multi backends (cinder::backend::*).
This commits updates the single volume backends to make use
of the respective multiple backends classes.

Change-Id: I26a796fb6131fa469f007af54cb96b3605c0f74d
Merge "multi backends: factorize code between single and multi backends"