The commit efab27afbf02743a3a2582e9a111eb1b7d985b26 in libvirt uses
/dev/%s for logical pools, change the tests to accommodate it.
(crobinso: additional fix to work with older libvirt)
We separate all the provisioning bits to diskbackend.py. VirtualDisk
users now need to explicitly opt in to storage creation by using
set_create_storage().
validation is no longer done automatically, users must call the
validation() command.
__init__ drops all extra parameters.
This will eventually get us to a point where we can unify the manual
XML building and XML parsing machinery, and get consistent validation
behavior between devices.
