Improve parsing of zfs volumes (ZVOLs) in order to avoid filtering of zvols nested under another ZFS dataset.

The actual code would only accept zvols like: POOL/vm-123-disk-1.
However, using POOL/DataSet/vm-123-disk-1 allows setting specific
proparties at POOL/DataSet level (like compression, etc.) which
would be inherited by any zvol created under such DataSet.

This allows more flexibility of zfs/zvol's management.

Signed-off-by: Pablo Ruiz García <pablo.ruiz@gmail.com>