gdbstub: processes initialization on new peer connection

gdbstub: processes initialization on new peer connection

When a new connection is established, we set the first process to be
attached, and the others detached. The first CPU of the first process
is selected as the current CPU.

Signed-off-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181207090135.7651-14-luc.michel@greensocs.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>