restapi: Fix not able to update non guid ids

Fixed the REST infrastructure to be able to accept
non guid ids in "update"  action of entities - use generic T instead of Guid.
This change aligns the "update" behavior  with the "add" action that
already works this way and allows non guid ids.

Change-Id: I4dbdca3fc820e7480d0bcffcec35568ee96b887d
Signed-off-by: Alissa Bonas <abonas@redhat.com>