Fix xenopsd common.py's VIF.get_ethtool() passing two args to list().append()

[bernhardkaindl]

Follow-up of #5921 that includes #5921 (please merge first) as it depends on it:

• Two commits are from xenopsd: Fix pytype warning on get_words: Name string is not defined #5921 and two commits (on get_ethtool()) are for this PR.

For @stephenchengCloud:
FYI: @ashwin9390

In #5896 (review), changes were requested:

Before merging feature/py3 into master, certain warnings in files that weren't checked so far should be fixed.

This PR fixes two of the mentioned pytype warnings:

pytype ocaml/xenopsd/scripts/common.py
File "ocaml/xenopsd/scripts/common.py", line 163, in get_ethtool: Function list.append expects 2 arg(s), got 3 [wrong-arg-count]
         Expected: (self, object)
  Actually passed: (self, object, _)
File "ocaml/xenopsd/scripts/common.py", line 165, in get_ethtool: Function list.append expects 2 arg(s), got 3 [wrong-arg-count]
         Expected: (self, object)
  Actually passed: (self, object, _)

These are from these lines:

                if v == "true" or v == "on":
                    results.append(k, True)
                elif v == "false" or v == "off":
                    results.append(k, False)

These ethtool settings are documented in the document Develop for XenServer starting at page 75.

As can be seen on the blame view, these two arguments have been there for 12 years. Maybe they worked in Python versions before Python2.7, but they did not even work in Python2.7.

Reaching such append(a, b) would raise a TypeError with any Interpreter that I can find:

python2 -c '[].append(1, True)'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: append() takes exactly one argument (2 given)

This PR adds a test to cover the method containing them and then fixes the bug.

The only call site of VIF().get_ethtool() is at https://github.com/xapi-project/xen-api/blob/feature/py3/ocaml/xenopsd/scripts/common.py#L229.

It expects that get_ethtool() returns a list of key/value sets like so: [(k, True), (k, False)], so pass (and return) k, bool as a set: .append((k, bool)).

[bernhardkaindl]

Closing in favour of #5923 which deletes the broken VIF.get_ethtool() code:

VIF.get_ethtool() was only used by Interface.online() which would be removed as the only user I could find was qemu-vif-script but that instantiated Interface() using only one argument, while it now takes 3 arguments.

This means that qemu-vif-script cannot work and as apparently nothing references it, we can thus remove the code only used by it. So I suggest removing the old code in #5923 instead.