Virtio console: Windows fixes

Hi guys,

I'm sending few windows-related virtio_console bugfixes and improvements. Please take a look on related messages.

Kind regards,
Lukáš

_Tested on F17@jeos qemu-1.4.0 using `run -t qemu --tests='virtio_console..loopback' -g WinXP.i386.sp3 --smp=4 --disk-bus=ide --qemu-bin=/usr/local/bin/qemu-system-x86_64 --nettype=bridge --no=spread_linear`_