virttest.virsh: add blkdeviotune command wrapper

Signed-off-by: Alex Jia <chuanchang.jia@gmail.com>
virttest.libvirt_xml.vm_xml: parse block device I/O elements from XML

Signed-off-by: Alex Jia <chuanchang.jia@gmail.com>
Merge pull request #1682 from chuanchang/blkdeviotune

Add helper functions for testing virsh blkdeviotune command