Apparently the xen block device statistics moved from
"/sys/devices/xen-backend/vbd-%d-%d/statistics/%s"
to
"/sys/bus/xen-backend/devices/vbd-%d-%d/statistics/%s"
* src/xen/block_stats.c: try the extra path in case of failure to
find the statistics in /sys
Replace free(virBufferContentAndReset()) with virBufferFreeAndReset().
Update documentation and replace all remaining calls to free() with
calls to VIR_FREE(). Also add missing calls to virBufferFreeAndReset()
and virReportOOMError() in OOM error cases.