Fix volume not found error message

The message 'KCHVOL0002E' expects the storage pool name, however the
backend was passing the libvirt pool object, this makes the user see the
python object string (useless information). This patch corrects this
problem, fixing the duplicated variable name in the source code.

Signed-off-by: Rodrigo Trujillo <rodrigo.trujillo@linux.vnet.ibm.com>