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 <[email protected]>
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 <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: KONRAD Frederic <[email protected]>
Message-id: 1521538773[email protected]
Signed-off-by: Peter Maydell <[email protected]>