tcg/plugins: implement a qemu_plugin_user_exit helper

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 <[email protected]>
Reviewed-by: Mahmoud Mandour <[email protected]>
Acked-by: Warner Losh <[email protected]>
Message-Id: <20210720232703[email protected]>