gdbstub: add multiprocess extension support

Add multiprocess extension support by enabling multiprocess mode when
the peer requests it, and by replying that we actually support it in the
qSupported reply packet.

Signed-off-by: Luc Michel <luc.michel@greensocs.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 20181207090135.7651-16-luc.michel@greensocs.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

gdbstub.c

@@ -1732,6 +1732,12 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf)
             if (cc->gdb_core_xml_file != NULL) {
                 pstrcat(buf, sizeof(buf), ";qXfer:features:read+");
             }
+
+            if (strstr(p, "multiprocess+")) {
+                s->multiprocess = true;
+            }
+            pstrcat(buf, sizeof(buf), ";multiprocess+");
+
             put_packet(s, buf);
             break;
         }