xen: constify all instances of "struct attribute_group"

The functions these get passed to have been taking pointers to const
since at least 2.6.16.

Signed-off-by: Jan Beulich <[e-mail]>
Signed-off-by: Konrad Rzeszutek Wilk <[e-mail]>

drivers/xen/sys-hypervisor.c

···
 	NULL
 };
 
-static struct attribute_group version_group = {
+static const struct attribute_group version_group = {
 	.name = "version",
 	.attrs = version_attrs,
 };
···
 	NULL
 };
 
-static struct attribute_group xen_compilation_group = {
+static const struct attribute_group xen_compilation_group = {
 	.name = "compilation",
 	.attrs = xen_compile_attrs,
 };
···
 	NULL
 };
 
-static struct attribute_group xen_properties_group = {
+static const struct attribute_group xen_properties_group = {
 	.name = "properties",
 	.attrs = xen_properties_attrs,
 };

drivers/xen/xen-balloon.c

···
 	NULL
 };
 
-static struct attribute_group balloon_info_group = {
+static const struct attribute_group balloon_info_group = {
 	.name = "info",
 	.attrs = balloon_info_attrs
 };

drivers/xen/xen-selfballoon.c

···
 	NULL
 };
 
-static struct attribute_group selfballoon_group = {
+static const struct attribute_group selfballoon_group = {
 	.name = "selfballoon",
 	.attrs = selfballoon_attrs
 };