xentop: add support for qdisks

xentop: add support for qdisks

Now that Xen uses qdisks by default and qemu does not write out
statistics to sysfs this patch queries the QMP for disk statistics.

This patch depends on libyajl for parsing statistics returned from
QMP. The runtime requires libyajl 2.0.3 or newer for required bug
fixes in yajl_tree_parse().

Libxl is modified to create a new socket dedicated for the use of
libxenstat for querying the block statistics using QMP.

The current APIs remain unchanged. It works within the existing
framework of libxenstat.

Signed-off-by: Charles Arnold <carnold@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>