Add volume migration code to Nexenta iSCSI volume driver

Utilize ZFS-specific optimizations for direct host-to-host
volume migration

DocImpact
Implements: blueprint nexenta-iscsi-volume-migrate
Change-Id: Ib9f2fa7e1ccb93dbdbc951ddc3a0eda55cb001d5
Merge "Add volume migration code to Nexenta iSCSI volume driver"