broaden the input parameter type for Stanza in function log_stanza
broaden the input parameter type for Stanza in function log_stanza

The prev type `exml:element` made dialyzer narrow the return type of
`escalus_connection:get_stanza` and warnings where emitted for
lines which were correctly executed in runtime.

See https://travis-ci.org/esl/escalus/jobs/165476937#L219 for details.

esl/escalus