iotests: add qmp recursive sorting function

Python before 3.6 does not sort dictionaries (including kwargs).
Therefore, printing QMP objects involves sorting the keys to have
a predictable ordering in the iotests output. This means that
iotests output will sometimes show arguments in an order not
specified by the test author.

Presently, we accomplish this by using json.dumps' sort_keys argument,
where we only serialize the arguments dictionary, but not the command.

However, if we want to pretty-print QMP objects being sent to the
QEMU process, we need to build the entire command before logging it.
Ordinarily, this would then involve "arguments" being sorted above
"execute", which would necessitate a rather ugly and harder-to-read
change to many iotests outputs.

To facilitate pretty-printing AND maintaining predictable output AND
having "arguments" sort after "execute", add a custom sort function
that takes a dictionary and recursively builds an OrderedDict that
maintains the specific key order we wish to see in iotests output.

The qmp_log function uses this to build a QMP object that keeps
"execute" above "arguments", but sorts all keys and keys in any
subdicts in "arguments" lexicographically to maintain consistent
iotests output, with no incompatible changes to any current test.

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: <20181221093529.23855-8-jsnow@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>