Add test for virsh blockcopy

Tracking issue
https://github.com/autotest/virt-test/issues/234

Introduce virsh blockcopy testing:

    1. Positive testing
        1.1 Copy a disk to a new image file.
        1.2 Reuse existing destination copy.
        1.3 Valid timeout and bandwidth test.
        1.4 Other valid options test
    2. Negative testing
        2.1 Copy a disk to a non-exist file.
        2.2 Copy a disk with invalid options.
        2.3 Do blcok copy for a persistent domain.

BTW, for max bandwidth testing, there's a bug(BZ#927156), and for invalid
timeout testing, there's a bug(BZ#927495).