accel/whpx: Implement missing AccelClass::gdbstub_supported_sstep_flags

Correct gdbstub support requires some gdbstub_supported_sstep_flags.
Apparently missed in commit d7482ffe975 ("whpx: Added support for
breakpoints and stepping"), even with the recent 19b48084f71 ("whpx:
i386: re-enable guest debug support") fixes.

Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Reviewed-by: Daniel Henrique Barboza <daniel.barboza@oss.qualcomm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20260705215729.62196-9-philmd@oss.qualcomm.com>