virsh: fixing segfault by pool autocompleter function.

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>