migration: reduce the count of strlen call 'strlen' is called three times in 'save_page_header', it's inefficient. Signed-off-by: Liang Li <liang.z.li@intel.com> Reviewed-by: Juan Quintela <quintela@redhat.com> Reviewed-by: Amit Shah <amit.shah@redhat.com> Signed-off-by: Juan Quintela <quintela@redhat.com>