libvirt.tests: add test cases for virsh blkdeviotune

    Test blkdevio tuning

    Positive test has covered the following combination.
    -------------------------
    | total | read  | write |
    -------------------------
    |   0   |   0   |   0   |
    | non-0 |   0   |   0   |
    |   0   | non-0 | non-0 |
    |   0   | non-0 |  0    |
    |   0   |   0   | non-0 |
    -------------------------

    Negative test has covered unsupported combination and
    invalid command arguments.

    NB: only qemu-kvm-rhev supports block I/O throttling on >= RHEL6.5,
    the qemu-kvm is okay for block I/O throttling on >= RHEL7.0.


BTW, This PR is based on https://github.com/autotest/virt-test/pull/1682
Oh, the The `post-test` is a must condition, the `pre-test` is up to test case, but for now there is no case  will alter libvirtd, and other testcases need running libvirtd, so the `pre-test` is a must condition..