xen/domain: Annotate struct domain as page aligned

struct domain is always a page aligned allocation.  Update it's type to
reflect this, so we can safely reuse the lower bits in the pointer for
auxiliary information.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/include/xen/sched.h

@@ -651,7 +651,7 @@ struct domain
     unsigned int num_llc_colors;
     const unsigned int *llc_colors;
 #endif
-};
+} __aligned(PAGE_SIZE);
 
 static inline struct page_list_head *page_to_list(
     struct domain *d, const struct page_info *pg)