restapi: introducing the instance types rest part

Implementation of the REST of the instance types feature:
http://www.ovirt.org/Features/Instance_Types

Since the instance types are implemented as templates, this patch heavily uses
the templates' mappers and model objects.

This patch both adds CRUD of the instance types as well as the add/edit/remove
of the instance type on the VM.

Change-Id: I45f464339e30258860f100240f34d1aa51e533ee
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>