migration: Create include for migration snapshots

Start removing migration code from sysemu/sysemu.h.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
diff --git a/include/block/block_int.h b/include/block/block_int.h
index e5eb473..cb78c4f 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -601,8 +601,8 @@
     int copy_on_read;
 
     /* If we are reading a disk image, give its size in sectors.
-     * Generally read-only; it is written to by load_vmstate and save_vmstate,
-     * but the block layer is quiescent during those.
+     * Generally read-only; it is written to by load_snapshot and
+     * save_snaphost, but the block layer is quiescent during those.
      */
     int64_t total_sectors;