virsh: Refactor block job waiting in cmdBlockPull
    
    Introduce helper function that will provide logic for waiting for block
    job completion so the 3 open coded places can be unified and improved.
    
    This patch introduces the whole logic and uses it to fix
    cmdBlockJobPull. The vshBlockJobWait function provides common logic for
    block job waiting that should be robust enough to work across all
    previous versions of libvirt. Since virsh allows passing user-provided
    strings as paths of block devices we can't reliably use block job events
    for detection of block job states so the function contains a great deal
    of fallback logic.