GetDiskByDiskId query support for user-level API

This patch adds support for the GetDiskByDiskId query for the user-level
API.

Change-Id: I83c873f3f427c10d8d9b56279c7b0a46299db287
Bug-Url: https://bugzilla.redhat.com/910092
Signed-off-by: Oved Ourfali <oourfali@redhat.com>