Fix: bt2: read properties on _DiscardedEventsMessage

Fix: bt2: read properties on _DiscardedEventsMessage

Reading the count and clock snapshot properties on a _DiscardedEventsMessage
does not work.  For example, this:

    msg = self._create_discarded_events_message(stream, count=10)
    print(msg.count)

Results in:

      File "test.py", line 10, in __init__
        print(msg.count)
      File "/home/simark/build/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/message.py", line 208, in count
        avail, count = self._get_count(self._ptr)
    AttributeError: '_DiscardedEventsMessage' object has no attribute '_get_count'

The problem is simply that _DiscardedEventsMessage is missing inheriting
from _DiscardedEventsMessageConst.

Change-Id: Id40ea54bb26b4e4aec264d84bda52ed815090341
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3392
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>