virtio_console: Modify virtio_console_guest.py guest to support Windows

This patch enables virtio_console_guest.py script to run on Windows.
It's a first very limited version.

VirtioGuest class now contains all functions, which might be used in
virtio_console tests and should be implemented.

The Posix version is the original Linux version with minor bugfixes.

The Nt version is simplified version which currently doesn't support
port reconnection (migration) and implements only "open" and "close"
features.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>