libxl: fix assertion failure in stub domain creation

libxl: fix assertion failure in stub domain creation

An assertion in libxl__domain_make():

'soft_reset || *domid == INVALID_DOMID'

does not hold true for stub domain creation, where soft_reset is false
but the passed in domid == 0. This is easily fixed by changing the
initializer in libxl__spawn_stub_dm().

NOTE: The comment for XEN_DOMCTL_createdomain in domctl.h is changed to
      reflect reality.

Fixes: 75259239d85d ("libxl_create: make 'soft reset' explicit")
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Wei Liu <wl@xen.org>
Acked-by: Jan Beulich <jbeulich@suse.com>