qga: Improve error report by calling error_setg_win32()

Use error_setg_win32() which adds a hint similar to strerror(errno)).

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200228100726.8414-4-philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>