libxl: libxl__domain_config_setdefault: New function

libxl: libxl__domain_config_setdefault: New function

Break out this into a new function.  We are going to want to call it
from a new call site.

Unfortunately not all of the defaults can be moved into the new
function without changing the order in which things are done.  That
does not seem wise at this stage of the release.  The effect is that
additional calls to libxl__domain_config_setdefault (which are going
to be introduced) do not quite set everything.  But they will do what
is needed.  After Xen 4.13 is done, we should move those settings into
the right order.

No functional change.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>