Fix free_vg order

Fix free_vg order

As now the FID management is more complex, the code inside free_vg needs
to access some parts of memory pools which were not needed before.

For this - makes the order of unlock_and_free_vg() unconditional.
Keek using unlock_and_free_vg() API function.

For properly working VG locking mechanism only the alphabeting locking
orderer needs to be preserved.

TODO: there could be few more code parts simplified when we 'officially'
support of referencies between different memory pools.