lxd/device/disk: Adds support to use ceph: prefix for disk source for VMs
lxd/device/disk: Adds support to use ceph: prefix for disk source for VMs

Utilises qemu's built in RBD support.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

lxc/lxd,lxc/lxd,hallyn/lxd,Skarlso/lxd,Skarlso/lxd,atxwebs/lxd,hallyn/lxd,stgraber/lxd,mjeanson/lxd,lxc/lxd,stgraber/lxd,mjeanson/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,atxwebs/lxd,lxc/lxd,atxwebs/lxd,Skarlso/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,atxwebs/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,mjeanson/lxd,Skarlso/lxd