[Bug Fix] Use object and parent id values for detecting root object

Change the way to identify root object.

We now use both object and parent ids values.

Object with a "0" object id value or a "-1" parent id value will
be considered as root object.

Reasons for accepting both:
a) there's code out there where root parent_id != -1 and
b) there's also code out there where root-like (parentless)
   objects id != 0.

Object with a "0" id value or a "-1" parent id value

This fixes the case of issue #32 where root have an empty string as
parent id value.

According to the specification root parent id must have a "-1" value.

Fix issue: https://github.com/01org/dleyna-server/issues/32

Signed-off-by: Christophe Guiraud <christophe.guiraud@intel.com>