restapi: add disk description mapping

This patch adds the disk description to the disk mapper so that it'll
be passed from the rest to the backend.

Change-Id: I42f5509b06363adfc15427d540e704d5f0cef5ec
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1056245
Signed-off-by: Liron Aravot <laravot@redhat.com>