pylibmount: use libmount reference counting

Signed-off-by: Karel Zak <kzak@redhat.com>
libmount: cleanup libmnt_fs list after mnt_table_remove_fs()

.. otherwise mnt_free_fs() will try to remove FS from non-existing
list.

Signed-off-by: Karel Zak <kzak@redhat.com>
pylibmount: add debug messages

Signed-off-by: Karel Zak <kzak@redhat.com>