engine: Fix updating of "shareable" property for direct LUNs

Currently the update of "shareable" property for direct LUNs is broken
due to ClassCastException as described in BZ993117.

Change-Id: Ie7b0e7c53b7a877b850106f8ea0f99026cee96e3
Bug-Url: https://bugzilla.redhat.com/993117
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>