iotests: Re-add filename filters

iotests: Re-add filename filters

A previous commit removed the default filters for qmp_log with the
intention to make them explicit; but this happened only for test 206.
There are more tests (for more exotic image formats than qcow2) which
require the filename filter, though.

Note that 237 is still broken for Python 2.x, which is fixed in the next
commit.

Fixes: f8ca8609d8549def45b28e82ecac64adaeee9f12
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20190210145736.1486-2-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>