gdbstub: eliminate gdbserver_fd global

We don't really need to track this fd beyond the initial creation of
the socket. We already know if the system has been initialised by
virtue of the gdbserver_state so lets remove it. This makes the later
re-factoring easier.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200430190122.4592-6-alex.bennee@linaro.org>