python-lvm: Implement proper refcounting for parent objects

python-lvm: Implement proper refcounting for parent objects

Our object nesting:

lib -> VG -> LV -> lvseg
          -> PV -> pvseg

Implement refcounting and checks to ensure parent objects are not
dealloced before their children. Also ensure calls to self or child's
methods are handled cleanly for objects that have been closed or removed.

Ensure all functions that are object methods have a first parameter named
'self', for consistency

Rename local vars that reference a Python object to '*obj', in order to
differentiate from liblvm handles

Fix a misspelled pv method name

Signed-off-by: Andy Grover <agrover@redhat.com>