lisa.trace: Cleanup event specification

* Remove support for space-separated string of events, since it already accepts
lists.
* Remove multiple assignments to the same attribute in __init__
* Accept any collections.abc.Sequence instead of just lists
* Make a copy of any sequence into a list(), so we don't modify objects
given by the user inplace. That is important since we sometimes pass the
list of events from TestBundle.ftrace_conf, which is supposed to be
read-only.