Basic stuff to allocate disks on different VGs

Extend allocation functions to be aware of different vg names.  Suppose,
vg_name is helded in disk.physical_id[0] (already was) LUCreateInstance
is modified to be aware of new "vg" argument of the disk specification.

cfg.GetVGName() will only be used when no VG is specified.

The "add" and "remove" for plain and drbd instances should be correct.
The "move" for plain instance should be correct.

Signed-off-by: Dmitry Chernyak <dmi.chernyak@gmail.com>
[iustin@google.com: fixed missing parameters in two cases and tabs]
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>