cmdDomblkinfo: add --all to show all block devices info
    
    This patch introduces --all to show all block devices info
    of guests like:
    
    virsh # domblkinfo w08 --all
    Target     Capacity        Allocation      Physical
    ---------------------------------------------------
    hda        42949672960     9878110208      9878110208
    vda        10737418240     10736439296     10737418240
    
    Target     Capacity        Allocation      Physical
    ---------------------------------------------------
    hda        40.000 GiB      9.200 GiB       9.200 GiB
    vda        10.000 GiB      9.999 GiB       10.000 GiB
    
    For inactive domains using networked storage, a "-" will
    be printed instead of the value since it's not possible
    to determine the value without the storage connection.
    
    Signed-off-by: Chen Hanxiao <chenhanxiao@gmail.com>
    Reviewed-by: John Ferlan <jferlan@redhat.com>