tcg/plugins: implement a qemu_plugin_user_exit helper

In user-mode emulation there is a small race between preexit_cleanup
and exit_group() which means we may end up calling instrumented
instructions before the kernel reaps child threads. To solve this we
implement a new helper which ensures the callbacks are flushed along
with any translations before we let the host do it's a thing.

While we are at it make the documentation of
qemu_plugin_register_atexit_cb clearer as to what the user can expect.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Mahmoud Mandour <ma.mandourr@gmail.com>
Acked-by: Warner Losh <imp@bsdimp.com>
Message-Id: <20210720232703.10650-21-alex.bennee@linaro.org>

linux-user/exit.c

@@ -35,5 +35,5 @@ void preexit_cleanup(CPUArchState *env, int code)
         __gcov_dump();
 #endif
         gdb_exit(code);
-        qemu_plugin_atexit_cb();
+        qemu_plugin_user_exit();
 }