Define the xml document style for transferring branch trace data.

Define the xml document style for transferring branch trace data.

Add a function to parse a btrace xml document into a vector of branch trace
blocks.

gdb/
	* features/btrace.dtd: New file.
	* Makefile.in (XMLFILES): Add btrace.dtd.
	* btrace.h (parse_xml_btrace): New declaration.
	* btrace.c: Include xml-support.h.
	(parse_xml_btrace): New function.
	(parse_xml_btrace_block): New function.
	(block_attributes): New struct.
	(btrace_attributes): New struct.
	(btrace_children): New struct.
	(btrace_elements): New struct.