btrace, python: Enable ptwrite filter registration.

By default GDB will be printing the hex payload of the ptwrite package as
auxiliary information.  To customize this, the user can register a ptwrite
filter function in python, that takes the payload and the PC as arguments and
returns a string which will be printed instead.  Registering the filter
function is done using a factory pattern to make per-thread filtering easier.

Approved-By: Markus Metzger <markus.t.metzger@intel.com>