x86/save: reserve HVM save record numbers that have been consumed...

...for patches not (yet) upstream.

This patch is simply adding a comment to reserve save record number space
to avoid the risk of clashes between existent downstream changes made by
Amazon and future upstream changes which may be incompatible.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Wei Liu <wl@xen.org>
Acked-by: Jan Beulich <jbeulich@suse.com>

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

@@ -639,6 +639,8 @@ struct hvm_msr {
 
 #define CPU_MSR_CODE  20
 
+/* Range 22 - 34 (inclusive) reserved for Amazon */
+
 /*
  * Largest type-code in use
  */