public: clarify xen_arch_domainconfig ABI statement

This structure is used by XEN_DOMCTL_createdomain, and is liable to be
modified going forwards.  Explicitly state that it falls under the
XEN_DOMCTL_INTERFACE_VERSION.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Julien Grall <julien.grall@citrix.com>
Reviewed-by: Don Slutz <dslutz@verizon.com>

xen/include/public/arch-arm.h

@@ -309,6 +309,10 @@ struct arch_shared_info {
 typedef struct arch_shared_info arch_shared_info_t;
 typedef uint64_t xen_callback_t;
 
+/*
+ * struct xen_arch_domainconfig's ABI is covered by
+ * XEN_DOMCTL_INTERFACE_VERSION.
+ */
 #define XEN_DOMCTL_CONFIG_GIC_DEFAULT   0
 #define XEN_DOMCTL_CONFIG_GIC_V2        1
 #define XEN_DOMCTL_CONFIG_GIC_V3        2

xen/include/public/arch-x86/xen.h

@@ -262,6 +262,10 @@ struct arch_shared_info {
 };
 typedef struct arch_shared_info arch_shared_info_t;
 
+/*
+ * struct xen_arch_domainconfig's ABI is covered by
+ * XEN_DOMCTL_INTERFACE_VERSION.
+ */
 struct xen_arch_domainconfig {
     char dummy;
 };