bt2: update value.py, make test_value pass

bt2: update value.py, make test_value pass

This patch updates value.py to match the current API.

The only known thing missing (to be done later) is to split classes in
their const and non-const variants (e.g. BoolValue and BoolValueConst),
where the methods that can modify values are only present on the
non-const versions.

The test is largely unchanged, except for the removal of things related
to frozen values.

Change-Id: I6424b0e6208ffe2cbe5e60fdd08579326abff29a
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1022
Tested-by: jenkins
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>