[EMULATORS] virtio_console: Add support for emergency write

Recently, there has been change in VirtIO specification for VirtIO console.

As per this change, a new "emerg_wr" or "emergency write" register has been
added to config space of VirtIO console. This new register is for debugging
purpose and GuestOS can use this register to print one charater at-a-time
without initializing VirtIO queues. The emerg_wr register can also be used
for earlyprintk at GuestOS boot time.

Also, the VirtIO specification only talks about write behaviour of the
"emerg_wr" register but nothing about read behaviour. In Xvisor, we treat
reads to "emerg_wr" as "emergency reads". If GuestOS reads this register
and some input character is available then we return (0x1<<31) | <input_char>
else we return 0x0.

This patch implements above described "emerg_wr" register to VirtIO Console
config space.

Signed-off-by: Anup Patel <anup@brainfault.org>