Allow a zfs pool or dataset as source for zfs storage backend

Allow a zfs pool or dataset as source for zfs storage backend

Enables hosting a pool on an existing zfs pool without affecting
other datasets there.
Specify dataset instead of pool as source to use.
Parent of dataset must exist for pool-build to succeed.
Beware that pool-delete destroys the source dataset and all children.

Solves: https://www.redhat.com/archives/libvirt-users/2017-April/msg00041.html

Signed-off-by: Gregor Kopka <gregor@kopka.net>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>