iotests: change qmp_log filters to expect QMP objects only

iotests: change qmp_log filters to expect QMP objects only

As laid out in the previous commit's message:

```
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.
```

Therefore:

Change qmp_log to treat filters as if they're always qmp object filters,
then change the logging call to rely on log()'s ability to serialize QMP
objects, so we're not duplicating that effort.

Add a qmp version of filter_testfiles and adjust the only caller using
it for qmp_log to use the qmp version.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20181221093529.23855-10-jsnow@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>