pool_create: Adjust location of --file pool

As of commit id 'c5f6c685' in virt-install (v1.0.0), virt-install
will attempt to create a storage pool for ease of management of
volumes. Thus if the test was started with --install or by chance
an 'images' pool exists using get_data_dir(), then the attempt
to define/create this pool will fail since two pools cannot use
the same path.  Therefore, rather than use the data dir for our
test, let's just use 'tmp' dir and create an images pool there.