iotests.py: Filter filename in any string value

filter_qmp_testfiles() currently filters the filename only for specific
keys.  However, there are more keys that take filenames (such as
block-commit's @top and @base, or ssh's @path), and it does not make
sense to list them all here.  "$TEST_DIR/$PID-" should have enough
entropy not to appear anywhere randomly.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Message-id: 20190210145736.1486-8-mreitz@redhat.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

tests/qemu-iotests/iotests.py

@@ -289,7 +289,7 @@ def filter_testfiles(msg):
 
 def filter_qmp_testfiles(qmsg):
     def _filter(key, value):
-        if key == 'filename' or key == 'backing-file':
+        if is_str(value):
             return filter_testfiles(value)
         return value
     return filter_qmp(qmsg, _filter)