virsh: fixing segfault by pool autocompleter function.
    
    The commands which requires a pool to perform any action for a volume is
    throwing a segfault when you pass the volume name before a pool name or
    without the argument '--pool'.
    
    An example that works:
    virsh # vol-list loops-pool
     Name                 Path
    -------------------------------------------------------------------
     loop0                /mnt/loop0
    
    virsh # vol-info --pool loops-pool lo<TAB>
    
    An example that does not work:
    virsh # vol-list loops-pool
     Name                 Path
    -------------------------------------------------------------------
     loop0                /mnt/loop0
    
    virsh # vol-info lo<TAB>
    Segmentation Fault
    
    The example 'vol-info' can be executed as 'vol-info loop0 --pool
    loops-pool'. So, this commit fixes this problem when the arguments are
    inverted and avoids the segfault.
    
    Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>