Disconnect vbd from domU

[James Dingwall <james-xen@xxxxxxxxxxxxxx>]

Hi,

Is it possible to trigger the disconnection of a block device from the 
domU?  I've managed to close the device from the domU with the 
following command but it still remains connected to the domain.

echo vbd-51712 > /sys/bus/xen/drivers/vbd/unbind

What I'm trying to achieve is booting from a ceph rbd which is mapped 
in dom0 but in the initrd disconnect the block device and then mount 
the root filesystem directly from the ceph cluster rather than block 
back/front.  I think this should let me live migrate the machine 
without having to map the rbd device in every dom0.

Thanks,
James