Add helper functions for testing virsh blkdeviotune command

virttest/virsh.py: add blkdeviotune command wrapper
virttest/libvirt_xml/vm_xml.py: parse block device I/O elements from XML
I think you should add a parameter named `target` or `device`, because perhaps there are several disks in VM, and this function can only get the first disk's information, use `target` or `device` you can get the the what you want.