xen: sysfs: make kobj_type structure constant

Since commit ee6d3dd4

 ("driver core: make kobj_type constant.")
the driver core allows the usage of const struct kobj_type.

Take advantage of this to constify the structure definition to prevent
modification at runtime.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20230216-kobj_type-xen-v1-1-742423de7d71@weissschuh.net
Signed-off-by: Juergen Gross <jgross@suse.com>