We can invoke the shell history writing when we leave the QMPShell scope
instead of relying on atexit. Doing so may be preferable to avoid global
state being registered from within a class instead of from the
application logic directly.

Use QMP's context manager to hook this history saving at close time,
which gets invoked when we leave the context block.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 20210607200649.1840382-32-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>