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
`post-test`? I think it's pre-test for environment check before start test, and if it's post-test, we should raise `error.TestFail`, not `error.TestNAError`.The pre-test is a `must condition`, but the post-test is up to test case....and I checked the configuration, there is no test case for dead libvirtd for now, so we can ignore considering about the `test scenario` you mentioned.