libvirt.tests: Add test for virsh blockjob

By creating blockcopy job, this test cover all options of virsh blockjob command.

    1. Positive testing
        1.1 Query active block job for the specified disk.
        1.2 Manager the active block job(cancle/pivot).
        1.3 Adjust speed for the active block job.
    2. Negative testing
        2.1 Query active block job for a invalid disk.
        2.2 Invalid bandwith test.
        2.3 No active block job management.

Signed-off-by: Yanbing Du <ydu@redhat.com>