A few bits and pieces to support the vhd daemon

A few bits and pieces to support the vhd daemon

 * Introduce a dereferencing type to camldm so that PVs can be referenced by ID rather than by device name. This is useful for passing a device-mapper table between hosts where the physical device for a particular PV is different.
 * Implement device-mapper reload/suspend/resume so that a table can be changed without tearing down the device
 * A few more helper functions for stdext

Signed-off-by: Jon Ludlam <Jonathan.Ludlam@eu.citrix.com>