simpletrace: made Analyzer into context-manager

simpletrace: made Analyzer into context-manager

Instead of explicitly calling `begin` and `end`, we can change the class
to use the context-manager paradigm. This is mostly a styling choice,
used in modern Python code. But it also allows for more advanced analyzers
to handle exceptions gracefully in the `__exit__` method (not
demonstrated here).

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Mads Ynddal <m.ynddal@samsung.com>
Message-id: 20230926103436.25700-9-mads@ynddal.dk
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>