gdb.trace: Save XML target description in tfile.

gdb/ChangeLog:

	* ctf.c (ctf_write_tdesc): New function.
	(ctf_write_ops): Wire in ctf_write_tdesc.
	* tracefile-tfile.c (tfile_write_tdesc): New function.
	(tfile_write_ops): Wire in tfile_write_tdesc.
	* tracefile.c (trace_save): Call write_tdesc method.
	* tracefile.h (struct trace_file_write_ops): Add write_tdesc method.
	* xml-tdesc.c (target_fetch_description_xml): New function.
	* xml-tdesc.h: Add target_fetch_description_xml prototype.