block: block: introduce APIs for submitting IO as a batch This patch introduces three APIs so that following patches can support queuing I/O requests and submitting them as a batch for improving I/O performance. Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Ming Lei <ming.lei@canonical.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>