wxx: Fix handling of files used for character devices

wxx: Fix handling of files used for character devices

On Windows, such files were not truncated like on all other hosts.
Now we also test whether truncation is needed when running on Windows.

The append case was also incorrect because it needs a different value
for the desired access mode.

Reported-by: Benjamin David Lunt <fys@fysnet.net>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1470114877-1466-1-git-send-email-sw@weilnetz.de
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>