bt2: make can_seek_beginning a method instead of a property

bt2: make can_seek_beginning a method instead of a property

_GenericMessageIterator.can_seek_beginning and
_UserMessageIterator._user_can_seek_beginning are currently properties.
We will add the equivalent for can_seek_ns_from_origin, but this one
requires a parameter (the ns_from_origin value), so it can't be a
property, it has to be a method.  For consistency, change the
can_seek_beginning ones to be methods.

Change-Id: Ic2c5b54908753dc7498fb1d0f94022301f766a15
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2017
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>