gdb.trace: Read XML target description from tfile.

gdb/ChangeLog:

	* tracefile-tfile.c (trace_tdesc): New static variable.
	(tfile_open): Clear trace_tdesc, call target_find_description.
	(tfile_interp_line): Recognize tdesc lines.
	(tfile_close): Clear trace_tdesc.
	(tfile_xfer_partial_features): New function.
	(tfile_xfer_partial): Call tfile_xfer_partial_features.
	(tfile_append_tdesc_line): New function.