Inconsistency when deleting volume attached to iSCSI target

[gcooper]

Deleting an iSCSI target with a volume that's currently in use warns the user that it's in use instead of providing a "are you sure you wish to delete this extent", kind of error. This is inconsistent with deleting volumes, ZFS volumes, and datasets, where the user is given an option to cascade delete all shares associated with the volume(s).

Procedure:

1. Go to "Storage" -> "Volumes" -> "Create Volume".
2. Add a volume of type "ZFS" named "test".
3. Go to "Create ZFS Volume".
4. Create a ZFS volume named "test2" using the "test" ZFS pool.
5. Go to "iSCSI" -> "Device Extent".
6. Add an extent named "test3", pointing to the ZFS volume "test2".
7. Go to "View all Volumes".
8. Try and delete "test2" (this will fail citing that the ZFS volume is in use by "test3").
9. Try and delete "test" zpool. Click "Yes" (this will succeed).

Expected result:

The user should be presented with a dialog asking if they wish to disable all services associated with the "test2" ZFS volume (i.e. the "test3" extent). If they press no, nothing should occur. Otherwise, it should cascade down and delete the iSCSI extent as well.

Actual result:

The user is blocked from deleting the iSCSI extent, but not when deleting the "test" ZFS pool.