iotests: remove default filters from qmp_log

Several places in iotests deal with serializing objects into JSON
strings, but to add pretty-printing it seems desirable to localize
all of those cases.

log() seems like a good candidate for that centralized behavior.
log() can already serialize json objects, but when it does so,
it assumes filters=[] operates on QMP objects, not strings.

qmp_log currently operates by dumping outgoing and incoming QMP
objects into strings and filtering them assuming that filters=[]
are string filters.

To have qmp_log use log's serialization, qmp_log will need to
accept only qmp filters, not text filters.

However, only a single caller of qmp_log actually requires any
filters at all. I remove the default filter and add it explicitly
to the caller in preparation for refactoring qmp_log to use rich
filters instead.

test 206 is amended to name the filter explicitly and the default
is removed.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20181221093529.23855-9-jsnow@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>