libxl: make libxl__wait_for_device_model use libxl__spawn_starrting directly

libxl: make libxl__wait_for_device_model use libxl__spawn_starrting directly

Instead of indirecting via libxl_device_model_starting. This fixes a
segmentation fault using stubdomains where starting->for_spawn is
(validly) NULL because starting a stubdom doesn't need to spawn a
process.

Most callers of libxl__wait_for_device_model already pass NULL for
this variable (because they are not on the starting path) so on
libxl__confirm_device_model_startup needs to change.

Reported-by: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>