LVM: Activate source LV before cloning from it

LVM may be configured to not automatically activate
thin-provisioned LVs.

Ensure they are activated when performing a clone, otherwise
dd will fail as the device does not exist in /dev/mapper/.

Closes-Bug: #1252423

Change-Id: Ibcb946ffe7804b1976bf1b1863c48340c8cc7fd5
Merge "LVM: Activate source LV before cloning from it"