gdbstub: send a termination packet instead of crashing gdb

Since the commit:
commit 4486e89c
Author: Stefan Hajnoczi <stefanha@redhat.com>
Date:   Wed Mar 7 14:42:05 2018 +0000

    vl: introduce vm_shutdown()

GDB crashes when qemu exits (at least on sparc-softmmu):
Remote communication error.  Target disconnected.: Connection reset by peer.
Quitting: putpkt: write failed: Broken pipe.

So send a packet to exit GDB before we exit QEMU:
[Inferior 1 (Thread 0) exited normally]
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKONRAD Frederic <frederic.konrad@adacore.com>
Message-id: 1521538773-30802-1-git-send-email-frederic.konrad@adacore.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

gdbstub.c

@@ -2052,6 +2052,13 @@ int gdbserver_start(const char *device)
     return 0;
 }
 
+void gdbserver_cleanup(void)
+{
+    if (gdbserver_state) {
+        put_packet(gdbserver_state, "W00");
+    }
+}
+
 static void register_types(void)
 {
     type_register_static(&char_gdb_type_info);

include/exec/gdbstub.h

@@ -103,6 +103,8 @@ int gdbserver_start(int);
 int gdbserver_start(const char *port);
 #endif
 
+void gdbserver_cleanup(void);
+
 /**
  * gdb_has_xml:
  * This is an ugly hack to cope with both new and old gdb.

vl.c

@@ -4733,6 +4733,8 @@ int main(int argc, char **argv, char **envp)
 
     main_loop();
 
+    gdbserver_cleanup();
+
     /* No more vcpu or device emulation activity beyond this point */
     vm_shutdown();