Fixes backup with multiple volume backend

The backup service now keeps a dictionary of managers
for each type of volume backend. If multi backend is
not in use (and volume_type is None) the current
volume_driver is used.

Change-Id: I66e91f3ce2cffaf2e2d07edfddc08b1480e2a37c
Fixes: bug 1228223
Merge "Fixes backup with multiple volume backend"