virtio: console: Add emergency writeonly register to config space

This patch adds an emerg_wr register (writeonly) in config space
of virtio console device which can be used for debugging.

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