When visitor_input_test_init_internal()'s argument @ap is null, then
@json_string is interpreted literally, else it's gets %-escapes
interpolated.  This is awkward.

One caller always passes null @ap, and the others never do.  Lift the
building of the QObject into the callers, where it can be done without
such ambiguity.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <20180806065344.7103-10-armbru@redhat.com>