Documentation: virtio: Add emergency write (emerg_wr) config register in virtio console.

[ Enhanced and applied to Lyx not text version --RR ]

Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Signed-off-by: Anup Patel <anup.patel@linaro.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>