postcopy: Transmit ram size summary word

Replace the host page-size in the 'advise' command by a pagesize
summary bitmap; if the VM is just using normal RAM then
this will be exactly the same as before, however if they're using
huge pages they'll be different, and thus:
   a) Migration from/to old qemu's that don't understand huge pages
      will fail early.
   b) Migrations with different size RAMBlocks will also fail early.

This catches it very early; earlier than the detailed per-block
check in the next patch.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20170224182844.32452-2-dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/ram.c

@@ -600,6 +600,23 @@ static void migration_bitmap_sync_init(void)
     iterations_prev = 0;
 }
 
+/* Returns a summary bitmap of the page sizes of all RAMBlocks;
+ * for VMs with just normal pages this is equivalent to the
+ * host page size.  If it's got some huge pages then it's the OR
+ * of all the different page sizes.
+ */
+uint64_t ram_pagesize_summary(void)
+{
+    RAMBlock *block;
+    uint64_t summary = 0;
+
+    QLIST_FOREACH_RCU(block, &ram_list.blocks, next) {
+        summary |= block->page_size;
+    }
+
+    return summary;
+}
+
 static void migration_bitmap_sync(void)
 {
     RAMBlock *block;