Move create_cloned_volume() to LVMVolumeDriver.

The implementation of create_cloned_volume() that was in driver.py
relied on functions that aren't declared there, such as
_create_volume(), _copy_volume(), and _sizestr().  Moved the function
to LVMVolumeDriver in lvm.py, where those functions exist.

Change-Id: I5ab77a119b3af0a152a3b5e0c2ff6a5ce196ad0f
Fixes: bug #1112485
Merge "Move create_cloned_volume() to LVMVolumeDriver."