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

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>