Improve Python bindings

[joachimmetz]

• add support to retrieve creation time for dfvfs
• catch physical volume file IO pool not being set earlier:

IOError: pyvslvm_logical_volume_read_buffer: unable to read data.
libbfio_pool_seek_offset: invalid pool.
libvslvm_chunk_data_read: unable to seek chunk data offset: 8388608 (0x00800000).
libvslvm_io_handle_read_chunk_data: unable to read chunk data.
libfdata_vector_get_element_value_by_index: unable to read element data at offset: 0x00800000.
libfdata_vector_get_element_value_at_offset: unable to retrieve element: 0 value.
libvslvm_internal_logical_volume_read_buffer_from_file_io_pool: unable to retrieve chunk data at offset: 0x0000018e.
libvslvm_logical_volume_read_buffer: unable to read buffer from logical volume.