libxl: prepare for asynchronous writing of qemu save file

libxl: prepare for asynchronous writing of qemu save file

* Combine the various calls to libxl__device_model_savefile into one
  at the start of libxl__domain_suspend, storing the result in the
  dss.  Consequently a few functions take a dss instead of some or all
  of their other arguments.

* Make libxl__domain_save_device_model's API into an asynchronous
  style which takes a callback.  The function is, however, still
  synchronous; it will be made actually async in the next patch.

* Consequently make libxl__remus_domain_checkpoint_callback into an
  asynchronous callback.

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