chardev: don't exit() straight away on C-a x

While there are a number of uses in the code-base of the exit(0)
pattern it gets in the way of clean exit which can do all of it's
house-keeping. In particular it was reported that you can crash
plugins this way because TCG can still be running on other threads
when the atexit callback is called.

Use qmp_quit() instead which takes care of some housekeeping before
triggering the shutdown.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reported-by: Lukas Jünger <lukas.junger@greensocs.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20211026102234.3961636-19-alex.bennee@linaro.org>

stubs/qmp-quit.c

@@ -0,0 +1,8 @@
+#include "qemu/osdep.h"
+#include "qapi/qapi-commands-control.h"
+#include "qapi/qmp/dispatch.h"
+
+void qmp_quit(Error **errp)
+{
+    g_assert_not_reached();
+}