ZFS: Support sparse volumes

ZFS: Support sparse volumes

By default, `zfs create -V ...` reserves space for the entire volsize,
plus some extra (which attempts to account for overhead).

If `zfs create -s -V ...` is used instead, zvols are (fully) sparse.

A middle ground (partial allocation) can be achieved with
`zfs create -s -o refreservation=... -V ...`.  Both libvirt and ZFS
support this approach, so the ZFS storage backend should support it.

Signed-off-by: Richard Laager <rlaager@wiktel.com>