Initial support of de-serializing the KMI of a Linux Kernel Tree

Initial support of de-serializing the KMI of a Linux Kernel Tree

With this patch, kmidiff knows how to compare a serialized
corpus_group that represents a Kernel Module Interface (a .kmi file)
against either another serialized .kmi file, or against a kernel tree.

The patch extends the abixml reader to make it parse an
'abi-corpus-group' element.  To do that, the patch modifies
read_corpus_from_input to make it be capable of parsing several
'abi-corpus' in a row.  That modified function is then used by a new
read_corpus_group_from_input, which is itself used by the new public
entry points read_corpus_group_from_native_xml_file and
read_corpus_group_from_native_xml.

With that read_corpus_group_from_native_xml_file building block
function, the kmidiff program is modified so that it can take either
two directory trees, two .kmi files or one directory tree and one .kmi
file.

	* include/abg-libxml-utils.h (advance_to_next_sibling_element):
	Declare new function.
	* src/abg-libxml-utils.cc (go_to_next_sibling_element_or_stay)
	(advance_to_next_sibling_element): Define new functions.
	* include/abg-reader.h (read_corpus_group_from_input)
	(read_corpus_group_from_native_xml)
	(read_corpus_group_from_native_xml_file): Declare new functions.
	* src/abg-reader.cc (read_context::m_corpus_group): New data
	member.
	(read_context::{get_corpus_group, set_corpus_group}): Define new
	member functions.
	(read_translation_unit_from_input): Cleanup logic.
	(read_corpus_from_input): Don't assume that the document is
	starting with an 'abi-corpus' element.  Support the mode where a
	caller called the xmlTextReaderExpand function (and so we are
	given an expanded xmlNodePtr) and the mode where we need to use
	the xmlTextReader API to walk through the 'abi-corpus' element.
	Also, if we are building a corpus group, do not clear what used to
	be 'per-corpus' data.  That data must be shared by all the corpora
	of a given abi-corpus-group.
	(read_corpus_group_from_input, read_corpus_group_from_native_xml)
	(read_corpus_group_from_native_xml_file): Define new functions.
	* include/abg-tools-utils.h (FILE_TYPE_XML_CORPUS_GROUP): New
	enumerator of the file_type enum.
	* src/abg-tools-utils.cc (operator<<): In the overload for
	file_type, add a case for the new FILE_TYPE_XML_CORPUS_GROUP.
	(guess_file_type): Dectect abi-corpus-group xml element.
	* tools/abidiff.cc (adjust_diff_context_for_kmidiff): Define new
	static function.
	(main): Adjust to handle the new FILE_TYPE_XML_CORPUS_GROUP.  That
	is, compare two FILE_TYPE_XML_CORPUS_GROUP if they are present.
	* tools/abilint.cc (main): Likewise.
	* tools/kmidiff.cc (main): Detect that one of two .kmi files are
	passed.  In that case, load the .kmi file(s), build a corpus_group
	of it and use it in the comparison.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>