libvirt: Add virsh_blockcopy test

This command can copy a disk backing image chain to dest.
    1. Positive testing
        1.1 Copy a disk to a new image file.
        1.2 Reuse existing destination copy.
        1.3 Valid blockcopy timeout and bandwidth test.
    2. Negative testing
        2.1 Copy a disk to a non-exist directory.
        2.2 Copy a disk with invalid options.
        2.3 Do blcok copy for a persistent domain.

Signed-off-by: Yanbing Du <ydu@redhat.com>