libvirt.tests: add test cases for virsh blkiotune

BTW, this PR bases on https://github.com/autotest/virt-test/pull/1678
'virsh version' is not enough, e.g. libvirt-1.1.1-28.el7.x86_64 and libvirt-1.1.1-29.el7.x86_64, they are the same output for these 2 different libvirt version.

# virsh version
Compiled against library: libvirt 1.1.1
Using library: libvirt 1.1.1
Using API: QEMU 1.1.1
Running hypervisor: QEMU 1.5.3