Use dummy driver for volumes

It makes no sense to use the aufs or devicemapper drivers for volumes.
The aufs one is perhaps not a big problem, but the devicemapper one
certainly is. It will be unnecessarily using a dm
blockdevice-over-loopback with a limited size base FS.

This just hardcodes the driver to be the dummy, perhaps in the future
we can have other drivers that make sense for the volumes.