Add support for addressing backing stores by index
    
    Each backing store of a given disk is associated with a unique index
    (which is also formatted in domain XML) for easier addressing of any
    particular backing store. With this patch, any backing store can be
    addressed by its disk target and the index. For example, "vdc[4]"
    addresses the backing store with index equal to 4 of the disk identified
    by "vdc" target. Such shorthand can be used in any API in place for a
    backing file path:
    
        virsh blockcommit domain vda --base vda[3] --top vda[2]
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>