Add support for the vAttach packets. In multiprocess mode, GDB sends
them to attach to additional processes.

Signed-off-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181207090135.7651-13-luc.michel@greensocs.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>