implement vdisk_clone and vdisk_create_base

This is an iplementation for file base storage types.

changes compared to patches from Alexandre:

* use correct locking
* private find_free_diskname() with bug fixes
* changed names of new methods
* always refer to base volumes in volume names

Example volume names:

local:6000/base-6000-disk-9.raw
local:6000/base-6000-disk-9.raw/7000/vm-7000-disk-9.qcow2
local:6000/base-6000-disk-9.raw/7000/base-7000-disk-10.qcow2
add stubs for create_base and cone_image