ceph_volume: fix bug in `is_lv()`

ceph_volume: fix bug in `is_lv()`

This function makes the `ceph_volume` module be not idempotent in
containerized context because it tries to run a container and bindmount
directories that no longer exist.

In that case, the `lvs` command being executed returns something
different than `0` so we can't call `json.loads(out)['report'][0]['lv']`
since it might throw an python error.

The idea is to return `True` only if `rc` is equal to `0` and
`len(result)` is greater than `0`, which means the command matched an
LV.

Fixes: #6284
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit ed79bc7a4e38e257f37faac50a5a19c06711dd8d)