sandbox: Remove the old memory file later

When debugging sandbox it is sometimes annoying that the memory file is
deleted early on. If sandbox later crashes or we quit (using the
debugger), it is not possible to run it again with the same state since
the memory file is gone.

Remove the old memory file when sandbox exits, instead. Also add debugging
showing the memory filename.

Signed-off-by: Simon Glass <sjg@chromium.org>