Nexenta iSCSI driver: Refactor create_cloned_volume

Optimize volume cloning of Nexenta iSCSI driver, current implementation
with zfs send recv may take long time, because it requires data
transferring.

Change-Id: Iaa2eea608e09d0a4ac155b74f964b153de561f4c
Merge "Nexenta iSCSI driver: Refactor create_cloned_volume"