Hard-code a volume prefix of 'rbd:'

This makes qemu happy (at least the version in xen-c6/CentOS 6.4) but
is clearly bad in the long term because we are limited to ceph storage.

Causes bug #1

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Update to 0.9.2

Signed-off-by: David Scott <dave.scott@eu.citrix.com>