Merge pull request #1 from vuntz/multi_backend

Fixes for multi-backend
Do not show two validation errors related to raw disks
Fix syntax error
Add hint in UI to explain backend name
Fix "add backend" button

Don't make it too large (looks bad), add correct spacing and improve
label to be more explicit.
Reset "Add backend" form when clicking add button

We want to start fresh for the next backend.
Use consistent id for "add backend" form elements
Improve migration script to multi-backend

We really don't want to change the defaults by importing old values in
there. We only really care about the single backend that was active, and
that's what we import back.
Remove volume_driver attribute from volume_defaults

It's unneeded.
Move use_multi_backend attribute outside of volume_defaults

It's unrelated to the defaults, really.