libxl: reorder libxl_device unplug functions

libxl: reorder libxl_device unplug functions

This is a reorder of functions, no functional change. This is needed
because in future patches much code is added to libxl_device and it
needs to follow the usual ao operation scheme (prototypes, functions
and callbacks in order they should be called)

Signed-off-by: Roger Pau Monne <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>