gdbstub: send a termination packet instead of crashing gdb

gdbstub: send a termination packet instead of crashing gdb

Since the commit:
commit 4486e89c219c0d1b9bd8dfa0b1dd5b0d51ff2268
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: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: KONRAD Frederic <frederic.konrad@adacore.com>
Message-id: 1521538773-30802-1-git-send-email-frederic.konrad@adacore.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>