gdbstub: Fix target.xml response

It was failing to return target.xml after the first request.

Fixes: 56e534bd ("gdbstub: refactor get_feature_xml")
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20230912224107.29669-3-akihiko.odaki@daynix.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231009164104.369749-10-alex.bennee@linaro.org>