gdbstub: use a better signal when we halt for IO reasons

gdbstub: use a better signal when we halt for IO reasons

The gdb description GDB_SIGNAL_IO is "I/O possible" and by default gdb
will try and restart the guest, getting us nowhere. Report
GDB_SIGNAL_STOP instead which should at least halt the session at the
failure point.

Reviewed-by: Luis Machado <luis.machado@arm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231201093633.2551497-2-alex.bennee@linaro.org>