gdbstub: add multiprocess support to vCont packets

gdbstub: add multiprocess support to vCont packets

Add the gdb_first_attached_cpu() and gdb_next_attached_cpu() to iterate
over all the CPUs in currently attached processes.

Add the gdb_first_cpu_in_process() and gdb_next_cpu_in_process() to
iterate over CPUs of a given process.

Use them to add multiprocess extension support to vCont packets.

Signed-off-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20181207090135.7651-6-luc.michel@greensocs.com
[PMM: corrected checkpatch comment style nit]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>