ceph-volume: fix has_bluestore_label() function

ceph-volume: fix has_bluestore_label() function

When using vg/lv, this function throws an error like following:

```
 stderr: unable to read label for test_group/data-lv2: (2) No such file or directory
 stderr: 2020-02-04T21:03:32.153+0000 7fe091af4200 -1 bluestore(test_group/data-lv2) _read_bdev_label failed to open test_group/data-lv2: (2) No such file or directory
```

using `self.abspath` fixes this error.

Fixes: https://tracker.ceph.com/issues/43970
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>