Add API to allow updating a volume attachment to a server

While attaching a volume to a VM, parameters of the attachment are passed, i.e. the bus type, and the device name. Updating those parameters is not possible without detaching and re-attaching the volume with the new parameters, and updating the boot volume's parameter is not possible at all.
A new API needs to be added to update the bdm parameters, only while the VM is powered-off, so that on the next boot, the volumes will be attached using the new parameters.