lxd/device/disk: Use rbd syntax for rbd volumes
lxd/device/disk: Use rbd syntax for rbd volumes

This uses the rbd syntax instead of mounting the disk, which can then be
used to add the disk using the QEMU rbd driver.

Signed-off-by: Thomas Hipp <5f82c492b3b00e427412d216ce820707a10c51ce@canonical.com>

hallyn/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd,hallyn/lxd