record-btrace: provide target_find_new_threads method

record-btrace: provide target_find_new_threads method

The "info threads" command tries to read memory, which is not possible during
replay.  This results in an error message and aborts the command without showing
the existing threads.

Provide a to_find_new_threads target method to skip the search while replaying.

2014-01-16  Markus Metzger  <markus.t.metzger@intel.com>

	* record-btrace.c (record_btrace_find_new_threads)
	(record_btrace_thread_alive): New.
	(init_record_btrace_ops): Initialize to_find_new_threads and
	to_thread_alive.